diff --git a/analytics/views.py b/analytics/views.py
index d9147ae3b2..619a9f3f8c 100644
--- a/analytics/views.py
+++ b/analytics/views.py
@@ -243,7 +243,7 @@ def get_chart_data(request: HttpRequest, user_profile: UserProfile, chart_name:
         labels_sort_function = None
         include_empty_subgroups = True
     else:
-        raise JsonableError(_("Unknown chart name: %s") % (chart_name,))
+        raise JsonableError(_("Unknown chart name: {}").format(chart_name))
 
     # Most likely someone using our API endpoint. The /stats page does not
     # pass a start or end in its requests.
@@ -252,8 +252,9 @@ def get_chart_data(request: HttpRequest, user_profile: UserProfile, chart_name:
     if end is not None:
         end = convert_to_UTC(end)
     if start is not None and end is not None and start > end:
-        raise JsonableError(_("Start time is later than end time. Start: %(start)s, End: %(end)s") %
-                            {'start': start, 'end': end})
+        raise JsonableError(_("Start time is later than end time. Start: {start}, End: {end}").format(
+            start=start, end=end,
+        ))
 
     if realm is None:
         # Note that this value is invalid for Remote tables; be
diff --git a/corporate/lib/stripe.py b/corporate/lib/stripe.py
index 3113e4a4ae..c94051a8c3 100644
--- a/corporate/lib/stripe.py
+++ b/corporate/lib/stripe.py
@@ -135,7 +135,9 @@ def renewal_amount(plan: CustomerPlan, event_time: datetime) -> int:  # nocovera
 
 class BillingError(Exception):
     # error messages
-    CONTACT_SUPPORT = _("Something went wrong. Please contact %s.") % (settings.ZULIP_ADMINISTRATOR,)
+    CONTACT_SUPPORT = _("Something went wrong. Please contact {email}.").format(
+        email=settings.ZULIP_ADMINISTRATOR,
+    )
     TRY_RELOADING = _("Something went wrong. Please reload the page.")
 
     # description is used only for tests
diff --git a/corporate/views.py b/corporate/views.py
index fe1628ac81..22d7f086d6 100644
--- a/corporate/views.py
+++ b/corporate/views.py
@@ -1,6 +1,6 @@
 import logging
 from decimal import Decimal
-from typing import Any, Dict, Optional, Union, cast
+from typing import Any, Dict, Optional, Union
 
 import stripe
 from django.conf import settings
@@ -85,14 +85,17 @@ def payment_method_string(stripe_customer: stripe.Customer) -> str:
     if stripe_source is None:  # nocoverage
         return _("No payment method on file")
     if stripe_source.object == "card":
-        return _("%(brand)s ending in %(last4)s") % {
-            'brand': cast(stripe.Card, stripe_source).brand,
-            'last4': cast(stripe.Card, stripe_source).last4}
+        assert isinstance(stripe_source, stripe.Card)
+        return _("{brand} ending in {last4}").format(
+            brand=stripe_source.brand, last4=stripe_source.last4,
+        )
     # There might be one-off stuff we do for a particular customer that
     # would land them here. E.g. by default we don't support ACH for
     # automatic payments, but in theory we could add it for a customer via
     # the Stripe dashboard.
-    return _("Unknown payment method. Please contact %s.") % (settings.ZULIP_ADMINISTRATOR,)  # nocoverage
+    return _("Unknown payment method. Please contact {email}.").format(
+        email=settings.ZULIP_ADMINISTRATOR,
+    )  # nocoverage
 
 @has_request_variables
 def upgrade(request: HttpRequest, user: UserProfile,
diff --git a/tools/lib/capitalization.py b/tools/lib/capitalization.py
index 3386d47cfc..2e1fbc0452 100644
--- a/tools/lib/capitalization.py
+++ b/tools/lib/capitalization.py
@@ -171,7 +171,7 @@ SPLIT_BOUNDARY_REGEX = re.compile(fr'[{SPLIT_BOUNDARY}]')
 
 # Regexes which check capitalization in sentences.
 DISALLOWED_REGEXES = [re.compile(regex) for regex in [
-    r'^[a-z]',  # Checks if the sentence starts with a lower case character.
+    r'^[a-z](?!\})',  # Checks if the sentence starts with a lower case character.
     r'^[A-Z][a-z]+[\sa-z0-9]+[A-Z]',  # Checks if an upper case character exists
     # after a lower case character when the first character is in upper case.
 ]]
diff --git a/tools/semgrep.yml b/tools/semgrep.yml
index 5f4d2eafd8..d53f6eadbf 100644
--- a/tools/semgrep.yml
+++ b/tools/semgrep.yml
@@ -81,6 +81,14 @@ rules:
     severity: ERROR
     message: "Do not write a SQL injection vulnerability please"
 
+  - id: translated-format
+    languages: [python]
+    pattern-either:
+      - pattern: django.utils.translation.ugettext(... .format(...))
+      - pattern: django.utils.translation.ugettext(f"...")
+    severity: ERROR
+    message: "Format strings after translation, not before"
+
   - id: mutable-default-type
     languages: [python]
     pattern-either:
@@ -122,5 +130,6 @@ rules:
     pattern-either:
       - pattern: '"..." % ...'
       - pattern: '("...") % ...'
+      - pattern: django.utils.translation.ugettext(...) % ...
     severity: ERROR
     message: "Prefer f-strings or .format for string formatting"
diff --git a/zerver/lib/actions.py b/zerver/lib/actions.py
index e322d95461..b28656b329 100644
--- a/zerver/lib/actions.py
+++ b/zerver/lib/actions.py
@@ -1948,7 +1948,7 @@ def validate_recipient_user_profiles(user_profiles: Sequence[UserProfile],
     for user_profile in user_profiles:
         if (not user_profile.is_active and not user_profile.is_mirror_dummy and
                 not allow_deactivated) or user_profile.realm.deactivated:
-            raise ValidationError(_("'%s' is no longer using Zulip.") % (user_profile.email,))
+            raise ValidationError(_("'{email}' is no longer using Zulip.").format(email=user_profile.email))
         recipient_profiles_map[user_profile.id] = user_profile
         if not is_cross_realm_bot_email(user_profile.email):
             realms.add(user_profile.realm_id)
@@ -2132,14 +2132,16 @@ def check_schedule_message(sender: UserProfile, client: Client,
 
 def check_default_stream_group_name(group_name: str) -> None:
     if group_name.strip() == "":
-        raise JsonableError(_("Invalid default stream group name '%s'") % (group_name,))
+        raise JsonableError(_("Invalid default stream group name '{}'").format(group_name))
     if len(group_name) > DefaultStreamGroup.MAX_NAME_LENGTH:
-        raise JsonableError(_("Default stream group name too long (limit: %s characters)")
-                            % (DefaultStreamGroup.MAX_NAME_LENGTH,))
+        raise JsonableError(_("Default stream group name too long (limit: {} characters)").format(
+            DefaultStreamGroup.MAX_NAME_LENGTH,
+        ))
     for i in group_name:
         if ord(i) == 0:
-            raise JsonableError(_("Default stream group name '%s' contains NULL (0x00) characters.")
-                                % (group_name,))
+            raise JsonableError(_("Default stream group name '{}' contains NULL (0x00) characters.").format(
+                group_name,
+            ))
 
 def send_rate_limited_pm_notification_to_bot_owner(sender: UserProfile,
                                                    realm: Realm,
@@ -2194,19 +2196,19 @@ def send_pm_if_empty_stream(stream: Optional[Stream],
     }
     if stream is None:
         if stream_id is not None:
-            content = _("Your bot `%(bot_identity)s` tried to send a message to stream ID "
-                        "%(stream_id)s, but there is no stream with that ID.") % arg_dict
+            content = _("Your bot `{bot_identity}` tried to send a message to stream ID "
+                        "{stream_id}, but there is no stream with that ID.").format(**arg_dict)
         else:
             assert(stream_name is not None)
-            content = _("Your bot `%(bot_identity)s` tried to send a message to stream "
-                        "#**%(stream_name)s**, but that stream does not exist. "
-                        "Click [here](#streams/new) to create it.") % arg_dict
+            content = _("Your bot `{bot_identity}` tried to send a message to stream "
+                        "#**{stream_name}**, but that stream does not exist. "
+                        "Click [here](#streams/new) to create it.").format(**arg_dict)
     else:
         if num_subscribers_for_stream_id(stream.id) > 0:
             return
-        content = _("Your bot `%(bot_identity)s` tried to send a message to "
-                    "stream #**%(stream_name)s**. The stream exists but "
-                    "does not have any subscribers.") % arg_dict
+        content = _("Your bot `{bot_identity}` tried to send a message to "
+                    "stream #**{stream_name}**. The stream exists but "
+                    "does not have any subscribers.").format(**arg_dict)
 
     send_rate_limited_pm_notification_to_bot_owner(sender, realm, content)
 
@@ -2349,7 +2351,9 @@ def check_message(sender: UserProfile, client: Client, addressee: Addressee,
 
         error_msg = check_widget_content(widget_content)
         if error_msg:
-            raise JsonableError(_('Widgets: %s') % (error_msg,))
+            raise JsonableError(_('Widgets: {error_msg}').format(
+                error_msg=error_msg,
+            ))
 
     return {'message': message, 'stream': stream, 'local_id': local_id,
             'sender_queue_id': sender_queue_id, 'realm': realm,
@@ -3583,12 +3587,13 @@ def do_rename_stream(stream: Stream,
         sender,
         stream,
         Realm.STREAM_EVENTS_NOTIFICATION_TOPIC,
-        _('@_**%(user_name)s|%(user_id)d** renamed stream **%(old_stream_name)s** to '
-          '**%(new_stream_name)s**.') % {
-              'user_name': user_profile.full_name,
-              'user_id': user_profile.id,
-              'old_stream_name': old_name,
-              'new_stream_name': new_name},
+        _('@_**{user_name}|{user_id}** renamed stream **{old_stream_name}** to '
+          '**{new_stream_name}**.').format(
+            user_name=user_profile.full_name,
+            user_id=user_profile.id,
+            old_stream_name=old_name,
+            new_stream_name=new_name,
+        ),
     )
     # Even though the token doesn't change, the web client needs to update the
     # email forwarding address to display the correctly-escaped new name.
@@ -3734,7 +3739,7 @@ def lookup_default_stream_groups(default_stream_group_names: List[str],
             default_stream_group = DefaultStreamGroup.objects.get(
                 name=group_name, realm=realm)
         except DefaultStreamGroup.DoesNotExist:
-            raise JsonableError(_('Invalid default stream group %s') % (group_name,))
+            raise JsonableError(_('Invalid default stream group {}').format(group_name))
         default_stream_groups.append(default_stream_group)
     return default_stream_groups
 
@@ -3771,15 +3776,16 @@ def do_create_default_stream_group(realm: Realm, group_name: str,
     for stream in streams:
         if stream in default_streams:
             raise JsonableError(_(
-                "'%(stream_name)s' is a default stream and cannot be added to '%(group_name)s'")
-                % {'stream_name': stream.name, 'group_name': group_name})
+                "'{stream_name}' is a default stream and cannot be added to '{group_name}'",
+            ).format(stream_name=stream.name, group_name=group_name))
 
     check_default_stream_group_name(group_name)
     (group, created) = DefaultStreamGroup.objects.get_or_create(
         name=group_name, realm=realm, description=description)
     if not created:
-        raise JsonableError(_("Default stream group '%(group_name)s' already exists")
-                            % {'group_name': group_name})
+        raise JsonableError(_(
+            "Default stream group '{group_name}' already exists",
+        ).format(group_name=group_name))
 
     group.streams.set(streams)
     notify_default_stream_groups(realm)
@@ -3790,12 +3796,12 @@ def do_add_streams_to_default_stream_group(realm: Realm, group: DefaultStreamGro
     for stream in streams:
         if stream in default_streams:
             raise JsonableError(_(
-                "'%(stream_name)s' is a default stream and cannot be added to '%(group_name)s'")
-                % {'stream_name': stream.name, 'group_name': group.name})
+                "'{stream_name}' is a default stream and cannot be added to '{group_name}'",
+            ).format(stream_name=stream.name, group_name=group.name))
         if stream in group.streams.all():
             raise JsonableError(_(
-                "Stream '%(stream_name)s' is already present in default stream group '%(group_name)s'")
-                % {'stream_name': stream.name, 'group_name': group.name})
+                "Stream '{stream_name}' is already present in default stream group '{group_name}'",
+            ).format(stream_name=stream.name, group_name=group.name))
         group.streams.add(stream)
 
     group.save()
@@ -3806,8 +3812,8 @@ def do_remove_streams_from_default_stream_group(realm: Realm, group: DefaultStre
     for stream in streams:
         if stream not in group.streams.all():
             raise JsonableError(_(
-                "Stream '%(stream_name)s' is not present in default stream group '%(group_name)s'")
-                % {'stream_name': stream.name, 'group_name': group.name})
+                "Stream '{stream_name}' is not present in default stream group '{group_name}'",
+            ).format(stream_name=stream.name, group_name=group.name))
         group.streams.remove(stream)
 
     group.save()
@@ -3816,10 +3822,10 @@ def do_remove_streams_from_default_stream_group(realm: Realm, group: DefaultStre
 def do_change_default_stream_group_name(realm: Realm, group: DefaultStreamGroup,
                                         new_group_name: str) -> None:
     if group.name == new_group_name:
-        raise JsonableError(_("This default stream group is already named '%s'") % (new_group_name,))
+        raise JsonableError(_("This default stream group is already named '{}'").format(new_group_name))
 
     if DefaultStreamGroup.objects.filter(name=new_group_name, realm=realm).exists():
-        raise JsonableError(_("Default stream group '%s' already exists") % (new_group_name,))
+        raise JsonableError(_("Default stream group '{}' already exists").format(new_group_name))
 
     group.name = new_group_name
     group.save()
@@ -4172,9 +4178,9 @@ def do_update_message_flags(user_profile: UserProfile,
     valid_flags = [item for item in UserMessage.flags
                    if item not in UserMessage.NON_API_FLAGS]
     if flag not in valid_flags:
-        raise JsonableError(_("Invalid flag: '%s'") % (flag,))
+        raise JsonableError(_("Invalid flag: '{}'").format(flag))
     if flag in UserMessage.NON_EDITABLE_FLAGS:
-        raise JsonableError(_("Flag not editable: '%s'") % (flag,))
+        raise JsonableError(_("Flag not editable: '{}'").format(flag))
     flagattr = getattr(UserMessage.flags, flag)
 
     msgs = UserMessage.objects.filter(user_profile=user_profile,
@@ -4250,15 +4256,19 @@ def notify_topic_moved_streams(user_profile: UserProfile,
     if send_notification_to_new_thread:
         internal_send_stream_message(
             new_stream.realm, sender, new_stream, new_topic,
-            _("This topic was moved here from %(old_location)s by %(user)s")
-            % dict(old_location=old_topic_link, user=user_mention))
+            _("This topic was moved here from {old_location} by {user}").format(
+                old_location=old_topic_link, user=user_mention,
+            ),
+        )
 
     if send_notification_to_old_thread:
         # Send a notification to the old stream that the topic was moved.
         internal_send_stream_message(
             old_stream.realm, sender, old_stream, old_topic,
-            _("This topic was moved by %(user)s to %(new_location)s")
-            % dict(user=user_mention, new_location=new_topic_link))
+            _("This topic was moved by {user} to {new_location}").format(
+                user=user_mention, new_location=new_topic_link,
+            ),
+        )
 
 def get_user_info_for_message_updates(message_id: int) -> MessageUpdateUserInfoResult:
 
@@ -4992,8 +5002,8 @@ def estimate_recent_invites(realms: Iterable[Realm], *, days: int) -> int:
 def check_invite_limit(realm: Realm, num_invitees: int) -> None:
     '''Discourage using invitation emails as a vector for carrying spam.'''
     msg = _("You do not have enough remaining invites. "
-            "Please contact %s to have your limit raised. "
-            "No invitations were sent.") % (settings.ZULIP_ADMINISTRATOR,)
+            "Please contact {email} to have your limit raised. "
+            "No invitations were sent.").format(email=settings.ZULIP_ADMINISTRATOR)
     if not settings.OPEN_REALM_CREATION:
         return
 
@@ -5617,7 +5627,7 @@ def check_add_user_group(realm: Realm, name: str, initial_members: List[UserProf
         user_group = create_user_group(name, initial_members, realm, description=description)
         do_send_create_user_group_event(user_group, initial_members)
     except django.db.utils.IntegrityError:
-        raise JsonableError(_("User group '%s' already exists.") % (name,))
+        raise JsonableError(_("User group '{}' already exists.").format(name))
 
 def do_send_user_group_update_event(user_group: UserGroup, data: Dict[str, Any]) -> None:
     event = dict(type="user_group", op='update', group_id=user_group.id, data=data)
@@ -5628,7 +5638,7 @@ def do_update_user_group_name(user_group: UserGroup, name: str) -> None:
         user_group.name = name
         user_group.save(update_fields=['name'])
     except django.db.utils.IntegrityError:
-        raise JsonableError(_("User group '%s' already exists.") % (name,))
+        raise JsonableError(_("User group '{}' already exists.").format(name))
     do_send_user_group_update_event(user_group, dict(name=name))
 
 def do_update_user_group_description(user_group: UserGroup, description: str) -> None:
diff --git a/zerver/lib/addressee.py b/zerver/lib/addressee.py
index ec1c6fd7a9..b0b57c1a4d 100644
--- a/zerver/lib/addressee.py
+++ b/zerver/lib/addressee.py
@@ -18,7 +18,7 @@ def get_user_profiles(emails: Iterable[str], realm: Realm) -> List[UserProfile]:
         try:
             user_profile = get_user_including_cross_realm(email, realm)
         except UserProfile.DoesNotExist:
-            raise JsonableError(_("Invalid email '%s'") % (email,))
+            raise JsonableError(_("Invalid email '{}'").format(email))
         user_profiles.append(user_profile)
     return user_profiles
 
diff --git a/zerver/lib/email_validation.py b/zerver/lib/email_validation.py
index dc37305518..ca6f314e61 100644
--- a/zerver/lib/email_validation.py
+++ b/zerver/lib/email_validation.py
@@ -174,7 +174,7 @@ def get_existing_user_errors(
 
         if existing_user_profile.is_active:
             if verbose:
-                msg = _('%s already has an account') % (email,)
+                msg = _('{email} already has an account').format(email=email)
             else:
                 msg = _("Already has an account.")
         else:
diff --git a/zerver/lib/emoji.py b/zerver/lib/emoji.py
index df339df2f2..30a87049dd 100644
--- a/zerver/lib/emoji.py
+++ b/zerver/lib/emoji.py
@@ -52,7 +52,7 @@ def emoji_name_to_emoji_code(realm: Realm, emoji_name: str) -> Tuple[str, str]:
         return emoji_name, Reaction.ZULIP_EXTRA_EMOJI
     if emoji_name in name_to_codepoint:
         return name_to_codepoint[emoji_name], Reaction.UNICODE_EMOJI
-    raise JsonableError(_("Emoji '%s' does not exist") % (emoji_name,))
+    raise JsonableError(_("Emoji '{}' does not exist").format(emoji_name))
 
 def check_emoji_request(realm: Realm, emoji_name: str, emoji_code: str,
                         emoji_type: str) -> None:
diff --git a/zerver/lib/narrow.py b/zerver/lib/narrow.py
index e77f189130..948b8578b7 100644
--- a/zerver/lib/narrow.py
+++ b/zerver/lib/narrow.py
@@ -21,7 +21,7 @@ def check_supported_events_narrow_filter(narrow: Iterable[Sequence[str]]) -> Non
     for element in narrow:
         operator = element[0]
         if operator not in ["stream", "topic", "sender", "is"]:
-            raise JsonableError(_("Operator %s not supported.") % (operator,))
+            raise JsonableError(_("Operator {} not supported.").format(operator))
 
 def is_web_public_compatible(narrow: Iterable[Dict[str, str]]) -> bool:
     for element in narrow:
diff --git a/zerver/lib/push_notifications.py b/zerver/lib/push_notifications.py
index bc255d6b82..672932d4ab 100644
--- a/zerver/lib/push_notifications.py
+++ b/zerver/lib/push_notifications.py
@@ -238,15 +238,17 @@ def parse_gcm_options(options: Dict[str, Any], data: Dict[str, Any]) -> str:
         else:  # `'event': 'remove'`, presumably
             priority = 'normal'
     if priority not in ('normal', 'high'):
-        raise JsonableError(_("Invalid GCM option to bouncer: priority %r")
-                            % (priority,))
+        raise JsonableError(_(
+            "Invalid GCM option to bouncer: priority {!r}",
+        ).format(priority))
 
     if options:
         # We're strict about the API; there is no use case for a newer Zulip
         # server talking to an older bouncer, so we only need to provide
         # one-way compatibility.
-        raise JsonableError(_("Invalid GCM options to bouncer: %s")
-                            % (ujson.dumps(options),))
+        raise JsonableError(_(
+            "Invalid GCM options to bouncer: {}",
+        ).format(ujson.dumps(options)))
 
     return priority  # when this grows a second option, can make it a tuple
 
@@ -600,9 +602,9 @@ def get_apns_alert_subtitle(message: Message) -> str:
     On an iOS notification, this is the second bolded line.
     """
     if message.trigger == "mentioned":
-        return _("%(full_name)s mentioned you:") % dict(full_name=message.sender.full_name)
+        return _("{full_name} mentioned you:").format(full_name=message.sender.full_name)
     elif message.trigger == "wildcard_mentioned":
-        return _("%(full_name)s mentioned everyone:") % dict(full_name=message.sender.full_name)
+        return _("{full_name} mentioned everyone:").format(full_name=message.sender.full_name)
     elif message.recipient.type == Recipient.PERSONAL:
         return ""
     # For group PMs, or regular messages to a stream, just use a colon to indicate this is the sender.
diff --git a/zerver/lib/remote_server.py b/zerver/lib/remote_server.py
index cace1c9199..2f0338327e 100644
--- a/zerver/lib/remote_server.py
+++ b/zerver/lib/remote_server.py
@@ -77,7 +77,7 @@ def send_to_push_bouncer(method: str,
         if 'code' in result_dict and result_dict['code'] == 'INVALID_ZULIP_SERVER':
             # Invalid Zulip server credentials should email this server's admins
             raise PushNotificationBouncerException(
-                _("Push notifications bouncer error: %s") % (msg,))
+                _("Push notifications bouncer error: {}").format(msg))
         else:
             # But most other errors coming from the push bouncer
             # server are client errors (e.g. never-registered token)
diff --git a/zerver/lib/request.py b/zerver/lib/request.py
index 5ab4f73e7e..dcbe00d6cb 100644
--- a/zerver/lib/request.py
+++ b/zerver/lib/request.py
@@ -348,7 +348,7 @@ def has_request_variables(view_func: ViewFuncT) -> ViewFuncT:
                 try:
                     val = ujson.loads(val)
                 except Exception:
-                    raise JsonableError(_('Argument "%s" is not valid JSON.') % (post_var_name,))
+                    raise JsonableError(_('Argument "{}" is not valid JSON.').format(post_var_name))
 
                 error = param.validator(post_var_name, val)
                 if error:
diff --git a/zerver/lib/streams.py b/zerver/lib/streams.py
index cf96a5d9c5..8b06436493 100644
--- a/zerver/lib/streams.py
+++ b/zerver/lib/streams.py
@@ -115,12 +115,12 @@ def create_streams_if_needed(realm: Realm,
 
 def check_stream_name(stream_name: str) -> None:
     if stream_name.strip() == "":
-        raise JsonableError(_("Invalid stream name '%s'") % (stream_name,))
+        raise JsonableError(_("Invalid stream name '{}'").format(stream_name))
     if len(stream_name) > Stream.MAX_NAME_LENGTH:
-        raise JsonableError(_("Stream name too long (limit: %s characters).") % (Stream.MAX_NAME_LENGTH,))
+        raise JsonableError(_("Stream name too long (limit: {} characters).").format(Stream.MAX_NAME_LENGTH))
     for i in stream_name:
         if ord(i) == 0:
-            raise JsonableError(_("Stream name '%s' contains NULL (0x00) characters.") % (stream_name,))
+            raise JsonableError(_("Stream name '{}' contains NULL (0x00) characters.").format(stream_name))
 
 def subscribed_to_stream(user_profile: UserProfile, stream_id: int) -> bool:
     return Subscription.objects.filter(
@@ -177,7 +177,7 @@ def access_stream_for_send_message(sender: UserProfile,
         return
 
     # All other cases are an error.
-    raise JsonableError(_("Not authorized to send to stream '%s'") % (stream.name,))
+    raise JsonableError(_("Not authorized to send to stream '{}'").format(stream.name))
 
 def check_for_exactly_one_stream_arg(stream_id: Optional[int], stream: Optional[str]) -> None:
     if stream_id is None and stream is None:
@@ -276,14 +276,14 @@ def check_stream_name_available(realm: Realm, name: str) -> None:
     check_stream_name(name)
     try:
         get_stream(name, realm)
-        raise JsonableError(_("Stream name '%s' is already taken.") % (name,))
+        raise JsonableError(_("Stream name '{}' is already taken.").format(name))
     except Stream.DoesNotExist:
         pass
 
 def access_stream_by_name(user_profile: UserProfile,
                           stream_name: str,
                           allow_realm_admin: bool=False) -> Tuple[Stream, Recipient, Optional[Subscription]]:
-    error = _("Invalid stream name '%s'") % (stream_name,)
+    error = _("Invalid stream name '{}'").format(stream_name)
     try:
         stream = get_realm_stream(stream_name, user_profile.realm_id)
     except Stream.DoesNotExist:
@@ -345,7 +345,7 @@ def can_access_stream_history(user_profile: UserProfile, stream: Stream) -> bool
 
     if stream.is_history_public_to_subscribers():
         # In this case, we check if the user is subscribed.
-        error = _("Invalid stream name '%s'") % (stream.name,)
+        error = _("Invalid stream name '{}'").format(stream.name)
         try:
             (recipient, sub) = access_stream_common(user_profile, stream, error)
         except JsonableError:
@@ -440,8 +440,9 @@ def list_to_streams(streams_raw: Iterable[Mapping[str, Any]],
         if not user_profile.can_create_streams():
             raise JsonableError(_('User cannot create streams.'))
         elif not autocreate:
-            raise JsonableError(_("Stream(s) (%s) do not exist") % ", ".join(
-                stream_dict["name"] for stream_dict in missing_stream_dicts))
+            raise JsonableError(_("Stream(s) ({}) do not exist").format(
+                ", ".join(stream_dict["name"] for stream_dict in missing_stream_dicts),
+            ))
 
         # We already filtered out existing streams, so dup_streams
         # will normally be an empty list below, but we protect against somebody
@@ -458,7 +459,7 @@ def access_default_stream_group_by_id(realm: Realm, group_id: int) -> DefaultStr
     try:
         return DefaultStreamGroup.objects.get(realm=realm, id=group_id)
     except DefaultStreamGroup.DoesNotExist:
-        raise JsonableError(_("Default stream group with id '%s' does not exist.") % (group_id,))
+        raise JsonableError(_("Default stream group with id '{}' does not exist.").format(group_id))
 
 def get_stream_by_narrow_operand_access_unchecked(operand: Union[str, int], realm: Realm) -> Stream:
     """This is required over access_stream_* in certain cases where
diff --git a/zerver/lib/users.py b/zerver/lib/users.py
index 1db7309f84..5c9ebda2f3 100644
--- a/zerver/lib/users.py
+++ b/zerver/lib/users.py
@@ -78,12 +78,13 @@ def check_valid_bot_config(bot_type: int, service_name: str,
                 config_options = {c[1]: c[2] for c in integration.config_options}
                 break
         if not config_options:
-            raise JsonableError(_("Invalid integration '%s'.") % (service_name,))
+            raise JsonableError(_("Invalid integration '{}'.").format(service_name))
 
         missing_keys = set(config_options.keys()) - set(config_data.keys())
         if missing_keys:
-            raise JsonableError(_("Missing configuration parameters: %s") % (
-                missing_keys,))
+            raise JsonableError(_("Missing configuration parameters: {}").format(
+                missing_keys,
+            ))
 
         for key, validator in config_options.items():
             value = config_data[key]
@@ -196,12 +197,12 @@ def user_ids_to_users(user_ids: Sequence[int], realm: Realm) -> List[UserProfile
     found_user_ids = user_profiles_by_id.keys()
     missed_user_ids = [user_id for user_id in user_ids if user_id not in found_user_ids]
     if missed_user_ids:
-        raise JsonableError(_("Invalid user ID: %s") % (missed_user_ids[0],))
+        raise JsonableError(_("Invalid user ID: {}").format(missed_user_ids[0]))
 
     user_profiles = list(user_profiles_by_id.values())
     for user_profile in user_profiles:
         if user_profile.realm != realm:
-            raise JsonableError(_("Invalid user ID: %s") % (user_profile.id,))
+            raise JsonableError(_("Invalid user ID: {}").format(user_profile.id))
     return user_profiles
 
 def access_bot_by_id(user_profile: UserProfile, user_id: int) -> UserProfile:
diff --git a/zerver/lib/validator.py b/zerver/lib/validator.py
index 4eeda8c14e..985b40ac7b 100644
--- a/zerver/lib/validator.py
+++ b/zerver/lib/validator.py
@@ -56,7 +56,7 @@ def set_type_structure(type_structure: TypeStructure) -> Callable[[FuncT], Any]:
 @set_type_structure("str")
 def check_string(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, str):
-        return _('%s is not a string') % (var_name,)
+        return _('{var_name} is not a string').format(var_name=var_name)
     return None
 
 @set_type_structure("str")
@@ -78,7 +78,7 @@ def check_string_in(possible_values: Union[Set[str], List[str]]) -> Validator:
         if not_str is not None:
             return not_str
         if val not in possible_values:
-            return _("Invalid %s") % (var_name,)
+            return _("Invalid {var_name}").format(var_name=var_name)
         return None
 
     return validator
@@ -91,7 +91,7 @@ def check_capped_string(max_length: int) -> Validator:
     @set_type_structure("str")
     def validator(var_name: str, val: object) -> Optional[str]:
         if not isinstance(val, str):
-            return _('%s is not a string') % (var_name,)
+            return _('{var_name} is not a string').format(var_name=var_name)
         if len(val) > max_length:
             return _("{var_name} is too long (limit: {max_length} characters)").format(
                 var_name=var_name, max_length=max_length)
@@ -103,7 +103,7 @@ def check_string_fixed_length(length: int) -> Validator:
     @set_type_structure("str")
     def validator(var_name: str, val: object) -> Optional[str]:
         if not isinstance(val, str):
-            return _('%s is not a string') % (var_name,)
+            return _('{var_name} is not a string').format(var_name=var_name)
         if len(val) != length:
             return _("{var_name} has incorrect length {length}; should be {target_length}").format(
                 var_name=var_name, target_length=length, length=len(val))
@@ -117,17 +117,17 @@ def check_long_string(var_name: str, val: object) -> Optional[str]:
 @set_type_structure("date")
 def check_date(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, str):
-        return _('%s is not a string') % (var_name,)
+        return _('{var_name} is not a string').format(var_name=var_name)
     try:
         datetime.strptime(val, '%Y-%m-%d')
     except ValueError:
-        return _('%s is not a date') % (var_name,)
+        return _('{var_name} is not a date').format(var_name=var_name)
     return None
 
 @set_type_structure("int")
 def check_int(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, int):
-        return _('%s is not an integer') % (var_name,)
+        return _('{var_name} is not an integer').format(var_name=var_name)
     return None
 
 def check_int_in(possible_values: List[int]) -> Validator:
@@ -137,7 +137,7 @@ def check_int_in(possible_values: List[int]) -> Validator:
         if not_int is not None:
             return not_int
         if val not in possible_values:
-            return _("Invalid %s") % (var_name,)
+            return _("Invalid {var_name}").format(var_name=var_name)
         return None
 
     return validator
@@ -145,23 +145,23 @@ def check_int_in(possible_values: List[int]) -> Validator:
 @set_type_structure("float")
 def check_float(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, float):
-        return _('%s is not a float') % (var_name,)
+        return _('{var_name} is not a float').format(var_name=var_name)
     return None
 
 @set_type_structure("bool")
 def check_bool(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, bool):
-        return _('%s is not a boolean') % (var_name,)
+        return _('{var_name} is not a boolean').format(var_name=var_name)
     return None
 
 @set_type_structure("str")
 def check_color(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, str):
-        return _('%s is not a string') % (var_name,)
+        return _('{var_name} is not a string').format(var_name=var_name)
     valid_color_pattern = re.compile(r'^#([a-fA-F0-9]{3,6})$')
     matched_results = valid_color_pattern.match(val)
     if not matched_results:
-        return _('%s is not a valid hex color code') % (var_name,)
+        return _('{var_name} is not a valid hex color code').format(var_name=var_name)
     return None
 
 def check_none_or(sub_validator: Validator) -> Validator:
@@ -190,11 +190,12 @@ def check_list(sub_validator: Optional[Validator], length: Optional[int]=None) -
     @set_type_structure(type_structure)
     def f(var_name: str, val: object) -> Optional[str]:
         if not isinstance(val, list):
-            return _('%s is not a list') % (var_name,)
+            return _('{var_name} is not a list').format(var_name=var_name)
 
         if length is not None and length != len(val):
-            return (_('%(container)s should have exactly %(length)s items') %
-                    {'container': var_name, 'length': length})
+            return (_('{container} should have exactly {length} items').format(
+                container=var_name, length=length,
+            ))
 
         if sub_validator:
             for i, item in enumerate(val):
@@ -215,12 +216,13 @@ def check_dict(required_keys: Iterable[Tuple[str, Validator]]=[],
     @set_type_structure(type_structure)
     def f(var_name: str, val: object) -> Optional[str]:
         if not isinstance(val, dict):
-            return _('%s is not a dict') % (var_name,)
+            return _('{var_name} is not a dict').format(var_name=var_name)
 
         for k, sub_validator in required_keys:
             if k not in val:
-                return (_('%(key_name)s key is missing from %(var_name)s') %
-                        {'key_name': k, 'var_name': var_name})
+                return (_('{key_name} key is missing from {var_name}').format(
+                    key_name=k, var_name=var_name,
+                ))
             vname = f'{var_name}["{k}"]'
             error = sub_validator(vname, val[k])
             if error:
@@ -251,7 +253,7 @@ def check_dict(required_keys: Iterable[Tuple[str, Validator]]=[],
             optional_keys_set = {x[0] for x in optional_keys}
             delta_keys = set(val.keys()) - required_keys_set - optional_keys_set
             if len(delta_keys) != 0:
-                return _("Unexpected arguments: %s") % (", ".join(list(delta_keys)),)
+                return _("Unexpected arguments: {}").format(", ".join(list(delta_keys)))
 
         return None
 
@@ -280,17 +282,16 @@ def check_variable_type(allowed_type_funcs: Iterable[Validator]) -> Validator:
         for func in allowed_type_funcs:
             if not func(var_name, val):
                 return None
-        return _('%s is not an allowed_type') % (var_name,)
+        return _('{var_name} is not an allowed_type').format(var_name=var_name)
     return enumerated_type_check
 
 def equals(expected_val: object) -> Validator:
     @set_type_structure(f'equals("{str(expected_val)}")')
     def f(var_name: str, val: object) -> Optional[str]:
         if val != expected_val:
-            return (_('%(variable)s != %(expected_value)s (%(value)s is wrong)') %
-                    {'variable': var_name,
-                     'expected_value': expected_val,
-                     'value': val})
+            return (_('{variable} != {expected_value} ({value} is wrong)').format(
+                variable=var_name, expected_value=expected_val, value=val,
+            ))
         return None
     return f
 
@@ -313,7 +314,7 @@ def check_url(var_name: str, val: object) -> Optional[str]:
         validate(val)
         return None
     except ValidationError:
-        return _('%s is not a URL') % (var_name,)
+        return _('{var_name} is not a URL').format(var_name=var_name)
 
 @set_type_structure('str')
 def check_external_account_url_pattern(var_name: str, val: object) -> Optional[str]:
@@ -435,7 +436,7 @@ def check_string_or_int_list(var_name: str, val: object) -> Optional[str]:
         return None
 
     if not isinstance(val, list):
-        return _('%s is not a string or an integer list') % (var_name,)
+        return _('{var_name} is not a string or an integer list').format(var_name=var_name)
 
     return check_list(check_int)(var_name, val)
 
@@ -444,4 +445,4 @@ def check_string_or_int(var_name: str, val: object) -> Optional[str]:
     if isinstance(val, str) or isinstance(val, int):
         return None
 
-    return _('%s is not a string or integer') % (var_name,)
+    return _('{var_name} is not a string or integer').format(var_name=var_name)
diff --git a/zerver/lib/zcommand.py b/zerver/lib/zcommand.py
index f6909b347d..16beb63894 100644
--- a/zerver/lib/zcommand.py
+++ b/zerver/lib/zcommand.py
@@ -54,4 +54,4 @@ def process_zcommands(content: str, user_profile: UserProfile) -> Dict[str, Any]
                                             switch_command='fluid-width',
                                             setting='fluid_layout_width',
                                             setting_value=False))
-    raise JsonableError(_('No such command: %s') % (command,))
+    raise JsonableError(_('No such command: {}').format(command))
diff --git a/zerver/tests/test_realm_emoji.py b/zerver/tests/test_realm_emoji.py
index 4874fb1fa9..e73f18c266 100644
--- a/zerver/tests/test_realm_emoji.py
+++ b/zerver/tests/test_realm_emoji.py
@@ -211,7 +211,7 @@ class RealmEmojiTest(ZulipTestCase):
         with get_test_image_file('img.png') as fp:
             with self.settings(MAX_EMOJI_FILE_SIZE=0):
                 result = self.client_post('/json/realm/emoji/my_emoji', {'file': fp})
-        self.assert_json_error(result, 'Uploaded file is larger than the allowed limit of 0 MB')
+        self.assert_json_error(result, 'Uploaded file is larger than the allowed limit of 0 MiB')
 
     def test_upload_already_existed_emoji(self) -> None:
         self.login('iago')
diff --git a/zerver/tests/test_upload.py b/zerver/tests/test_upload.py
index fa0084658c..0afd388466 100644
--- a/zerver/tests/test_upload.py
+++ b/zerver/tests/test_upload.py
@@ -164,7 +164,7 @@ class FileUploadTest(UploadSerializeMixin, ZulipTestCase):
         # would be 1MB.
         with self.settings(MAX_FILE_UPLOAD_SIZE=0):
             result = self.client_post("/json/user_uploads", {'f1': fp})
-        self.assert_json_error(result, 'Uploaded file is larger than the allowed limit of 0 MB')
+        self.assert_json_error(result, 'Uploaded file is larger than the allowed limit of 0 MiB')
 
     def test_multiple_upload_failure(self) -> None:
         """
@@ -1163,7 +1163,7 @@ class AvatarTest(UploadSerializeMixin, ZulipTestCase):
         with get_test_image_file(self.correct_files[0][0]) as fp:
             with self.settings(MAX_AVATAR_FILE_SIZE=0):
                 result = self.client_post("/json/users/me/avatar", {'file': fp})
-        self.assert_json_error(result, "Uploaded file is larger than the allowed limit of 0 MB")
+        self.assert_json_error(result, "Uploaded file is larger than the allowed limit of 0 MiB")
 
     def tearDown(self) -> None:
         destroy_uploads()
@@ -1336,7 +1336,7 @@ class RealmIconTest(UploadSerializeMixin, ZulipTestCase):
         with get_test_image_file(self.correct_files[0][0]) as fp:
             with self.settings(MAX_ICON_FILE_SIZE=0):
                 result = self.client_post("/json/realm/icon", {'file': fp})
-        self.assert_json_error(result, "Uploaded file is larger than the allowed limit of 0 MB")
+        self.assert_json_error(result, "Uploaded file is larger than the allowed limit of 0 MiB")
 
     def tearDown(self) -> None:
         destroy_uploads()
@@ -1479,7 +1479,7 @@ class RealmLogoTest(UploadSerializeMixin, ZulipTestCase):
             with self.settings(MAX_LOGO_FILE_SIZE=0):
                 result = self.client_post("/json/realm/logo", {'file': fp, 'night':
                                                                ujson.dumps(self.night)})
-        self.assert_json_error(result, "Uploaded file is larger than the allowed limit of 0 MB")
+        self.assert_json_error(result, "Uploaded file is larger than the allowed limit of 0 MiB")
 
     def tearDown(self) -> None:
         destroy_uploads()
diff --git a/zerver/tornado/event_queue.py b/zerver/tornado/event_queue.py
index 14f8c40070..120303af7f 100644
--- a/zerver/tornado/event_queue.py
+++ b/zerver/tornado/event_queue.py
@@ -566,13 +566,17 @@ def fetch_events(query: Mapping[str, Any]) -> Dict[str, Any]:
                 client.event_queue.newest_pruned_id is not None
                 and last_event_id < client.event_queue.newest_pruned_id
             ):
-                raise JsonableError(_("An event newer than %s has already been pruned!") % (last_event_id,))
+                raise JsonableError(_("An event newer than {event_id} has already been pruned!").format(
+                    event_id=last_event_id,
+                ))
             client.event_queue.prune(last_event_id)
             if (
                 client.event_queue.newest_pruned_id is not None
                 and last_event_id != client.event_queue.newest_pruned_id
             ):
-                raise JsonableError(_("Event %s was not in this queue") % (last_event_id,))
+                raise JsonableError(_("Event {event_id} was not in this queue").format(
+                    event_id=last_event_id,
+                ))
             was_connected = client.finish_current_handler()
 
         if not client.event_queue.empty() or dont_block:
diff --git a/zerver/views/hotspots.py b/zerver/views/hotspots.py
index 929ee2692e..7a8d5f2abe 100644
--- a/zerver/views/hotspots.py
+++ b/zerver/views/hotspots.py
@@ -15,6 +15,6 @@ from zerver.models import UserProfile
 def mark_hotspot_as_read(request: HttpRequest, user: UserProfile,
                          hotspot: str=REQ(validator=check_string)) -> HttpResponse:
     if hotspot not in ALL_HOTSPOTS:
-        return json_error(_('Unknown hotspot: %s') % (hotspot,))
+        return json_error(_('Unknown hotspot: {}').format(hotspot))
     do_mark_hotspot_as_read(user, hotspot)
     return json_success()
diff --git a/zerver/views/messages.py b/zerver/views/messages.py
index e27f7ce0c3..9b82f9ad96 100644
--- a/zerver/views/messages.py
+++ b/zerver/views/messages.py
@@ -852,8 +852,9 @@ def get_messages_backend(request: HttpRequest, user_profile: UserProfile,
                          apply_markdown: bool=REQ(validator=check_bool, default=True)) -> HttpResponse:
     anchor = parse_anchor_value(anchor_val, use_first_unread_anchor_val)
     if num_before + num_after > MAX_MESSAGES_PER_FETCH:
-        return json_error(_("Too many messages requested (maximum %s).")
-                          % (MAX_MESSAGES_PER_FETCH,))
+        return json_error(_("Too many messages requested (maximum {}).").format(
+            MAX_MESSAGES_PER_FETCH,
+        ))
 
     if user_profile.realm.email_address_visibility != Realm.EMAIL_ADDRESS_VISIBILITY_EVERYONE:
         # If email addresses are only available to administrators,
@@ -1218,7 +1219,7 @@ def mark_topic_as_read(request: HttpRequest,
         )
 
         if not topic_exists:
-            raise JsonableError(_('No such topic \'%s\'') % (topic_name,))
+            raise JsonableError(_('No such topic \'{}\'').format(topic_name))
 
     count = do_mark_stream_messages_as_read(user_profile, request.client, stream, topic_name)
 
@@ -1413,7 +1414,7 @@ def send_message_backend(request: HttpRequest, user_profile: UserProfile,
         try:
             realm = get_realm(realm_str)
         except Realm.DoesNotExist:
-            return json_error(_("Unknown organization '%s'") % (realm_str,))
+            return json_error(_("Unknown organization '{}'").format(realm_str))
 
     if client.name in ["zephyr_mirror", "irc_mirror", "jabber_mirror", "JabberMirror"]:
         # Here's how security works for mirroring:
diff --git a/zerver/views/presence.py b/zerver/views/presence.py
index 7d7e23d6be..0bb30ece03 100644
--- a/zerver/views/presence.py
+++ b/zerver/views/presence.py
@@ -30,7 +30,7 @@ def get_presence_backend(request: HttpRequest, user_profile: UserProfile,
 
     presence_dict = get_presence_for_user(target.id)
     if len(presence_dict) == 0:
-        return json_error(_('No presence data for %s') % (email,))
+        return json_error(_('No presence data for {email}').format(email=email))
 
     # For initial version, we just include the status and timestamp keys
     result = dict(presence=presence_dict[target.email])
@@ -77,7 +77,7 @@ def update_active_status_backend(request: HttpRequest, user_profile: UserProfile
                                  ) -> HttpResponse:
     status_val = UserPresence.status_from_string(status)
     if status_val is None:
-        raise JsonableError(_("Invalid status: %s") % (status,))
+        raise JsonableError(_("Invalid status: {}").format(status))
     elif user_profile.presence_enabled:
         update_user_presence(user_profile, request.client, timezone_now(),
                              status_val, new_user_input)
diff --git a/zerver/views/realm.py b/zerver/views/realm.py
index 4297549310..91cef0e71d 100644
--- a/zerver/views/realm.py
+++ b/zerver/views/realm.py
@@ -94,7 +94,7 @@ def update_realm(
     # Additional validation/error checking beyond types go here, so
     # the entire request can succeed or fail atomically.
     if default_language is not None and default_language not in get_available_language_codes():
-        raise JsonableError(_("Invalid language '%s'") % (default_language,))
+        raise JsonableError(_("Invalid language '{}'").format(default_language))
     if description is not None and len(description) > 1000:
         return json_error(_("Organization description is too long."))
     if name is not None and len(name) > Realm.MAX_REALM_NAME_LENGTH:
diff --git a/zerver/views/realm_domains.py b/zerver/views/realm_domains.py
index 60d682ccb7..9c347bc522 100644
--- a/zerver/views/realm_domains.py
+++ b/zerver/views/realm_domains.py
@@ -26,8 +26,8 @@ def create_realm_domain(request: HttpRequest, user_profile: UserProfile,
     except ValidationError as e:
         return json_error(_('Invalid domain: {}').format(e.messages[0]))
     if RealmDomain.objects.filter(realm=user_profile.realm, domain=domain).exists():
-        return json_error(_("The domain %(domain)s is already"
-                            " a part of your organization.") % {'domain': domain})
+        return json_error(_("The domain {domain} is already"
+                            " a part of your organization.").format(domain=domain))
     realm_domain = do_add_realm_domain(user_profile.realm, domain, allow_subdomains)
     return json_success({'new_domain': [realm_domain.id, realm_domain.domain]})
 
@@ -39,7 +39,7 @@ def patch_realm_domain(request: HttpRequest, user_profile: UserProfile, domain:
         realm_domain = RealmDomain.objects.get(realm=user_profile.realm, domain=domain)
         do_change_realm_domain(realm_domain, allow_subdomains)
     except RealmDomain.DoesNotExist:
-        return json_error(_('No entry found for domain %(domain)s.') % {'domain': domain})
+        return json_error(_('No entry found for domain {domain}.').format(domain=domain))
     return json_success()
 
 @require_realm_admin
@@ -50,5 +50,5 @@ def delete_realm_domain(request: HttpRequest, user_profile: UserProfile,
         realm_domain = RealmDomain.objects.get(realm=user_profile.realm, domain=domain)
         do_remove_realm_domain(realm_domain)
     except RealmDomain.DoesNotExist:
-        return json_error(_('No entry found for domain %(domain)s.') % {'domain': domain})
+        return json_error(_('No entry found for domain {domain}.').format(domain=domain))
     return json_success()
diff --git a/zerver/views/realm_emoji.py b/zerver/views/realm_emoji.py
index 0758caceaa..911a702788 100644
--- a/zerver/views/realm_emoji.py
+++ b/zerver/views/realm_emoji.py
@@ -32,8 +32,9 @@ def upload_emoji(request: HttpRequest, user_profile: UserProfile,
         return json_error(_("You must upload exactly one file."))
     emoji_file = list(request.FILES.values())[0]
     if (settings.MAX_EMOJI_FILE_SIZE * 1024 * 1024) < emoji_file.size:
-        return json_error(_("Uploaded file is larger than the allowed limit of %s MB") % (
-            settings.MAX_EMOJI_FILE_SIZE))
+        return json_error(_("Uploaded file is larger than the allowed limit of {} MiB").format(
+            settings.MAX_EMOJI_FILE_SIZE,
+        ))
 
     realm_emoji = check_add_realm_emoji(user_profile.realm,
                                         emoji_name,
@@ -49,7 +50,7 @@ def delete_emoji(request: HttpRequest, user_profile: UserProfile,
     if not RealmEmoji.objects.filter(realm=user_profile.realm,
                                      name=emoji_name,
                                      deactivated=False).exists():
-        raise JsonableError(_("Emoji '%s' does not exist") % (emoji_name,))
+        raise JsonableError(_("Emoji '{}' does not exist").format(emoji_name))
     check_emoji_admin(user_profile, emoji_name)
     do_remove_realm_emoji(user_profile.realm, emoji_name)
     return json_success()
diff --git a/zerver/views/realm_export.py b/zerver/views/realm_export.py
index 29d7bdcc58..cf3aefb297 100644
--- a/zerver/views/realm_export.py
+++ b/zerver/views/realm_export.py
@@ -43,8 +43,9 @@ def export_realm(request: HttpRequest, user: UserProfile) -> HttpResponse:
                                                    property='messages_sent:client:day'))
     if (total_messages > MAX_MESSAGE_HISTORY or
             user.realm.currently_used_upload_space_bytes() > MAX_UPLOAD_QUOTA):
-        return json_error(_('Please request a manual export from %s.') % (
-            settings.ZULIP_ADMINISTRATOR,))
+        return json_error(_('Please request a manual export from {email}.').format(
+            email=settings.ZULIP_ADMINISTRATOR,
+        ))
 
     row = RealmAuditLog.objects.create(realm=realm,
                                        event_type=event_type,
diff --git a/zerver/views/realm_icon.py b/zerver/views/realm_icon.py
index 70dff5711b..cc6d4f44fb 100644
--- a/zerver/views/realm_icon.py
+++ b/zerver/views/realm_icon.py
@@ -20,8 +20,9 @@ def upload_icon(request: HttpRequest, user_profile: UserProfile) -> HttpResponse
 
     icon_file = list(request.FILES.values())[0]
     if ((settings.MAX_ICON_FILE_SIZE * 1024 * 1024) < icon_file.size):
-        return json_error(_("Uploaded file is larger than the allowed limit of %s MB") % (
-            settings.MAX_ICON_FILE_SIZE))
+        return json_error(_("Uploaded file is larger than the allowed limit of {} MiB").format(
+            settings.MAX_ICON_FILE_SIZE,
+        ))
     upload_icon_image(icon_file, user_profile)
     do_change_icon_source(user_profile.realm, user_profile.realm.ICON_UPLOADED)
     icon_url = realm_icon_url(user_profile.realm)
diff --git a/zerver/views/realm_logo.py b/zerver/views/realm_logo.py
index ea09ff8cd0..0641f6b395 100644
--- a/zerver/views/realm_logo.py
+++ b/zerver/views/realm_logo.py
@@ -24,8 +24,9 @@ def upload_logo(request: HttpRequest, user_profile: UserProfile,
         return json_error(_("You must upload exactly one logo."))
     logo_file = list(request.FILES.values())[0]
     if ((settings.MAX_LOGO_FILE_SIZE * 1024 * 1024) < logo_file.size):
-        return json_error(_("Uploaded file is larger than the allowed limit of %s MB") % (
-            settings.MAX_LOGO_FILE_SIZE))
+        return json_error(_("Uploaded file is larger than the allowed limit of {} MiB").format(
+            settings.MAX_LOGO_FILE_SIZE,
+        ))
     upload_logo_image(logo_file, user_profile, night)
     do_change_logo_source(user_profile.realm, user_profile.realm.LOGO_UPLOADED, night)
     return json_success()
diff --git a/zerver/views/streams.py b/zerver/views/streams.py
index 5fc30d6846..06281b62a8 100644
--- a/zerver/views/streams.py
+++ b/zerver/views/streams.py
@@ -356,12 +356,14 @@ def you_were_just_subscribed_message(acting_user: UserProfile,
                                      stream_names: Set[str]) -> str:
     subscriptions = sorted(list(stream_names))
     if len(subscriptions) == 1:
-        return _("@**%(full_name)s** subscribed you to the stream #**%(stream_name)s**.") % \
-            {"full_name": acting_user.full_name,
-             "stream_name": subscriptions[0]}
+        return _("@**{full_name}** subscribed you to the stream #**{stream_name}**.").format(
+            full_name=acting_user.full_name,
+            stream_name=subscriptions[0],
+        )
 
-    message = _("@**%(full_name)s** subscribed you to the following streams:") % \
-        {"full_name": acting_user.full_name}
+    message = _("@**{full_name}** subscribed you to the following streams:").format(
+        full_name=acting_user.full_name,
+    )
     message += "\n\n"
     for stream_name in subscriptions:
         message += f"* #**{stream_name}**\n"
@@ -416,7 +418,9 @@ def add_subscriptions_backend(
     authorized_streams, unauthorized_streams = \
         filter_stream_authorization(user_profile, existing_streams)
     if len(unauthorized_streams) > 0 and authorization_errors_fatal:
-        return json_error(_("Unable to access stream (%s).") % unauthorized_streams[0].name)
+        return json_error(_("Unable to access stream ({stream_name}).").format(
+            stream_name=unauthorized_streams[0].name,
+        ))
     # Newly created streams are also authorized for the creator
     streams = authorized_streams + created_streams
 
@@ -519,9 +523,10 @@ def add_subscriptions_backend(
                     sender=sender,
                     stream=stream,
                     topic=Realm.STREAM_EVENTS_NOTIFICATION_TOPIC,
-                    content=_('Stream created by @_**%(user_name)s|%(user_id)d**.') % {
-                        'user_name': user_profile.full_name,
-                        'user_id': user_profile.id},
+                    content=_('Stream created by @_**{user_name}|{user_id}**.').format(
+                        user_name=user_profile.full_name,
+                        user_id=user_profile.id,
+                    ),
                 ),
             )
 
@@ -675,11 +680,11 @@ def update_subscription_properties_backend(
         value = change["value"]
 
         if property not in property_converters:
-            return json_error(_("Unknown subscription property: %s") % (property,))
+            return json_error(_("Unknown subscription property: {}").format(property))
 
         (stream, recipient, sub) = access_stream_by_id(user_profile, stream_id)
         if sub is None:
-            return json_error(_("Not subscribed to stream id %d") % (stream_id,))
+            return json_error(_("Not subscribed to stream id {}").format(stream_id))
 
         property_conversion = property_converters[property](property, value)
         if property_conversion:
diff --git a/zerver/views/upload.py b/zerver/views/upload.py
index eaa724e29b..fb8df3cabe 100644
--- a/zerver/views/upload.py
+++ b/zerver/views/upload.py
@@ -107,8 +107,9 @@ def upload_file_backend(request: HttpRequest, user_profile: UserProfile) -> Http
     user_file = list(request.FILES.values())[0]
     file_size = user_file.size
     if settings.MAX_FILE_UPLOAD_SIZE * 1024 * 1024 < file_size:
-        return json_error(_("Uploaded file is larger than the allowed limit of %s MB") % (
-            settings.MAX_FILE_UPLOAD_SIZE))
+        return json_error(_("Uploaded file is larger than the allowed limit of {} MiB").format(
+            settings.MAX_FILE_UPLOAD_SIZE,
+        ))
     check_upload_within_quota(user_profile.realm, file_size)
 
     uri = upload_message_image_from_request(request, user_file, user_profile)
diff --git a/zerver/views/user_groups.py b/zerver/views/user_groups.py
index 82d408a472..5af07b854b 100644
--- a/zerver/views/user_groups.py
+++ b/zerver/views/user_groups.py
@@ -99,7 +99,9 @@ def add_members_to_group_backend(request: HttpRequest, user_profile: UserProfile
 
     for user_profile in user_profiles:
         if user_profile.id in existing_member_ids:
-            raise JsonableError(_("User %s is already a member of this group") % (user_profile.id,))
+            raise JsonableError(_("User {user_id} is already a member of this group").format(
+                user_id=user_profile.id,
+            ))
 
     bulk_add_members_to_user_group(user_group, user_profiles)
     return json_success()
@@ -114,7 +116,7 @@ def remove_members_from_group_backend(request: HttpRequest, user_profile: UserPr
     group_member_ids = get_user_group_members(user_group)
     for member in members:
         if (member not in group_member_ids):
-            raise JsonableError(_("There is no member '%s' in this user group") % (member,))
+            raise JsonableError(_("There is no member '{}' in this user group").format(member))
 
     remove_members_from_user_group(user_group, user_profiles)
     return json_success()
diff --git a/zerver/views/user_settings.py b/zerver/views/user_settings.py
index b19fe08ff3..9437b4ca17 100644
--- a/zerver/views/user_settings.py
+++ b/zerver/views/user_settings.py
@@ -95,7 +95,7 @@ def json_change_settings(request: HttpRequest, user_profile: UserProfile,
         except RateLimited as e:
             secs_to_freedom = int(float(str(e)))
             return json_error(
-                _("You're making too many attempts! Try again in %s seconds.") % (secs_to_freedom,),
+                _("You're making too many attempts! Try again in {} seconds.").format(secs_to_freedom),
             )
 
         if not check_password_strength(new_password):
@@ -221,7 +221,7 @@ def json_change_notify_settings(
 
     if (notification_sound is not None and
             notification_sound not in get_available_notification_sounds()):
-        raise JsonableError(_("Invalid notification sound '%s'") % (notification_sound,))
+        raise JsonableError(_("Invalid notification sound '{}'").format(notification_sound))
 
     req_vars = {k: v for k, v in list(locals().items()) if k in user_profile.notification_setting_types}
 
@@ -241,8 +241,9 @@ def set_avatar_backend(request: HttpRequest, user_profile: UserProfile) -> HttpR
 
     user_file = list(request.FILES.values())[0]
     if ((settings.MAX_AVATAR_FILE_SIZE * 1024 * 1024) < user_file.size):
-        return json_error(_("Uploaded file is larger than the allowed limit of %s MB") % (
-            settings.MAX_AVATAR_FILE_SIZE))
+        return json_error(_("Uploaded file is larger than the allowed limit of {} MiB").format(
+            settings.MAX_AVATAR_FILE_SIZE,
+        ))
     upload_avatar_image(user_file, user_profile, user_profile)
     do_change_avatar_fields(user_profile, UserProfile.AVATAR_FROM_USER)
     user_avatar_url = avatar_url(user_profile)
diff --git a/zerver/views/users.py b/zerver/views/users.py
index 2b88c66661..9c4db54fc5 100644
--- a/zerver/views/users.py
+++ b/zerver/views/users.py
@@ -479,8 +479,9 @@ def create_user_backend(request: HttpRequest, user_profile: UserProfile,
     try:
         email_allowed_for_realm(email, user_profile.realm)
     except DomainNotAllowedForRealmError:
-        return json_error(_("Email '%(email)s' not allowed in this organization") %
-                          {'email': email})
+        return json_error(_("Email '{email}' not allowed in this organization").format(
+            email=email,
+        ))
     except DisposableEmailError:
         return json_error(_("Disposable email addresses are not allowed in this organization"))
     except EmailContainsPlusError:
@@ -488,7 +489,7 @@ def create_user_backend(request: HttpRequest, user_profile: UserProfile,
 
     try:
         get_user_by_delivery_email(email, user_profile.realm)
-        return json_error(_("Email '%s' already in use") % (email,))
+        return json_error(_("Email '{}' already in use").format(email))
     except UserProfile.DoesNotExist:
         pass
 
diff --git a/zerver/webhooks/freshdesk/view.py b/zerver/webhooks/freshdesk/view.py
index 4156d64244..7c07f46c0f 100644
--- a/zerver/webhooks/freshdesk/view.py
+++ b/zerver/webhooks/freshdesk/view.py
@@ -146,7 +146,7 @@ def api_freshdesk_webhook(request: HttpRequest, user_profile: UserProfile,
         if ticket_data.get(key) is None:
             logging.warning("Freshdesk webhook error. Payload was:")
             logging.warning(request.body)
-            return json_error(_("Missing key %s in JSON") % (key,))
+            return json_error(_("Missing key {} in JSON").format(key))
 
     ticket = TicketDict(ticket_data)
 
diff --git a/zerver/webhooks/wordpress/view.py b/zerver/webhooks/wordpress/view.py
index bdf0898343..3901e4a65d 100644
--- a/zerver/webhooks/wordpress/view.py
+++ b/zerver/webhooks/wordpress/view.py
@@ -42,7 +42,7 @@ def api_wordpress_webhook(request: HttpRequest, user_profile: UserProfile,
         data = WP_LOGIN_TEMPLATE.format(name=user_login)
 
     else:
-        return json_error(_("Unknown WordPress webhook action: %s") % (hook,))
+        return json_error(_("Unknown WordPress webhook action: {}").format(hook))
 
     topic = 'WordPress Notification'
 
diff --git a/zilencer/views.py b/zilencer/views.py
index ad659c4f1c..26e6530688 100644
--- a/zilencer/views.py
+++ b/zilencer/views.py
@@ -74,7 +74,7 @@ def register_remote_server(
         url_validator = URLValidator()
         url_validator('http://' + hostname)
     except ValidationError:
-        raise JsonableError(_('%s is not a valid hostname') % (hostname,))
+        raise JsonableError(_('{} is not a valid hostname').format(hostname))
 
     try:
         validate_email(contact_email)
@@ -179,7 +179,7 @@ def validate_incoming_table_data(server: RemoteZulipServer, model: Any,
     last_id = get_last_id_from_server(server, model)
     for row in rows:
         if is_count_stat and row['property'] not in COUNT_STATS:
-            raise JsonableError(_("Invalid property %s") % (row['property'],))
+            raise JsonableError(_("Invalid property {}").format(row['property']))
         if row['id'] <= last_id:
             raise JsonableError(_("Data is out of order."))
         last_id = row['id']