api: Fix encoding of strings in user settings endpoints.

* Remove unnecessary json_validator for string parameters.
* Remove unnecessary JSON encoding in frontend calls.  Structurally,
  JavaScript does correct encoding without explicit JSON encoding.

Fixes part of #18035.

static/js/settings_display.js

@@ -90,7 +90,7 @@ export function set_up() {
 
         const $link = $(e.target).closest("a[data-code]");
         const setting_value = $link.attr("data-code");
-        const data = {default_language: JSON.stringify(setting_value)};
+        const data = {default_language: setting_value};
 
         const new_language = $link.attr("data-name");
         $("#default_language_name").text(new_language);

static/js/settings_notifications.js

@@ -42,9 +42,9 @@ function rerender_ui() {
     }
 }
 
-function change_notification_setting(setting, setting_data, status_element) {
+function change_notification_setting(setting, value, status_element) {
     const data = {};
-    data[setting] = JSON.stringify(setting_data);
+    data[setting] = value;
     settings_ui.do_settings_change(
         channel.patch,
         "/json/settings/notifications",

zerver/tests/test_settings.py

@@ -173,17 +173,17 @@ class ChangeSettingsTest(ZulipTestCase):
         user_profile = self.example_user("hamlet")
         self.login_user(user_profile)
 
-        json_result = self.client_patch(pattern, {param: orjson.dumps("invalid").decode()})
+        json_result = self.client_patch(pattern, {param: "invalid"})
         self.assert_json_error(json_result, "Invalid notification sound 'invalid'")
 
-        json_result = self.client_patch(pattern, {param: orjson.dumps("ding").decode()})
+        json_result = self.client_patch(pattern, {param: "ding"})
         self.assert_json_success(json_result)
 
         # refetch user_profile object to correctly handle caching
         user_profile = self.example_user("hamlet")
         self.assertEqual(getattr(user_profile, param), "ding")
 
-        json_result = self.client_patch(pattern, {param: orjson.dumps("zulip").decode()})
+        json_result = self.client_patch(pattern, {param: "zulip"})
 
         self.assert_json_success(json_result)
         # refetch user_profile object to correctly handle caching
@@ -353,6 +353,10 @@ class ChangeSettingsTest(ZulipTestCase):
             invalid_value: Any = 100
         else:
             invalid_value = "invalid_" + setting_name
+
+        if setting_name == "default_language":
+            data = {setting_name: test_value}
+        else:
             data = {setting_name: orjson.dumps(test_value).decode()}
 
         result = self.client_patch("/json/settings/display", data)
@@ -362,6 +366,9 @@ class ChangeSettingsTest(ZulipTestCase):
 
         # Test to make sure invalid settings are not accepted
         # and saved in the db.
+        if setting_name == "default_language":
+            data = {setting_name: invalid_value}
+        else:
             data = {setting_name: orjson.dumps(invalid_value).decode()}
 
         result = self.client_patch("/json/settings/display", data)

zerver/views/user_settings.py

@@ -42,7 +42,7 @@ from zerver.lib.request import JsonableError
 from zerver.lib.response import json_error, json_success
 from zerver.lib.send_email import FromAddress, send_email
 from zerver.lib.upload import upload_avatar_image
-from zerver.lib.validator import check_bool, check_int, check_int_in, check_string, check_string_in
+from zerver.lib.validator import check_bool, check_int, check_int_in, check_string_in
 from zerver.models import UserProfile, avatar_changes_disabled, name_changes_disabled
 from zproject.backends import check_password_strength, email_belongs_to_ldap
 
@@ -197,7 +197,7 @@ def update_display_settings_backend(
         json_validator=check_int_in(UserProfile.COLOR_SCHEME_CHOICES), default=None
     ),
     translate_emoticons: Optional[bool] = REQ(json_validator=check_bool, default=None),
-    default_language: Optional[str] = REQ(json_validator=check_string, default=None),
+    default_language: Optional[str] = REQ(default=None),
     default_view: Optional[str] = REQ(
         json_validator=check_string_in(default_view_options), default=None
     ),
@@ -243,7 +243,7 @@ def json_change_notify_settings(
         json_validator=check_bool, default=None
     ),
     wildcard_mentions_notify: Optional[bool] = REQ(json_validator=check_bool, default=None),
-    notification_sound: Optional[str] = REQ(json_validator=check_string, default=None),
+    notification_sound: Optional[str] = REQ(default=None),
     enable_desktop_notifications: Optional[bool] = REQ(json_validator=check_bool, default=None),
     enable_sounds: Optional[bool] = REQ(json_validator=check_bool, default=None),
     enable_offline_email_notifications: Optional[bool] = REQ(