i18n: Add get_browser_language_code function.

This commit adds get_browser_language_code function
which returns None if there is no Accept-language
header in the request or Accept-languge header contains
only unsupported languages or all languages (meaning
header having value of '*'). Otherwise it returns the
language with highest weight/quality-value.

zerver/lib/i18n.py

@@ -9,6 +9,7 @@ import orjson
 from django.conf import settings
 from django.http import HttpRequest
 from django.utils import translation
+from django.utils.translation.trans_real import parse_accept_lang_header
 
 from zerver.lib.request import RequestNotes
 
@@ -67,3 +68,17 @@ def get_and_set_request_language(
     RequestNotes.get_notes(request).set_language = translation.get_language()
 
     return request_language
+
+
+def get_browser_language_code(request: HttpRequest) -> Optional[str]:
+    accept_lang_header = request.META.get("HTTP_ACCEPT_LANGUAGE")
+    if accept_lang_header is None:
+        return None
+
+    available_language_codes = get_available_language_codes()
+    for accept_lang, priority in parse_accept_lang_header(request.META.get("HTTP_ACCEPT_LANGUAGE")):
+        if accept_lang == "*":
+            return None
+        if accept_lang in available_language_codes:
+            return accept_lang
+    return None

zerver/tests/test_i18n.py

@@ -9,7 +9,9 @@ from django.http import HttpResponse
 from django.utils import translation
 
 from zerver.lib.email_notifications import enqueue_welcome_emails
+from zerver.lib.i18n import get_browser_language_code
 from zerver.lib.test_classes import ZulipTestCase
+from zerver.lib.test_helpers import HostRequestMock
 from zerver.management.commands import makemessages
 from zerver.models import get_realm_stream
 
@@ -128,6 +130,28 @@ class TranslationTestCase(ZulipTestCase):
             response = self.fetch("get", f"/{lang}/integrations/", 200)
             self.assert_in_response(word, response)
 
+    def test_get_browser_language_code(self) -> None:
+        req = HostRequestMock()
+
+        self.assertIsNone(get_browser_language_code(req))
+
+        req.META["HTTP_ACCEPT_LANGUAGE"] = "de"
+        self.assertEqual(get_browser_language_code(req), "de")
+
+        req.META["HTTP_ACCEPT_LANGUAGE"] = "en-GB,en;q=0.8"
+        self.assertEqual(get_browser_language_code(req), "en-gb")
+
+        # Case when unsupported language has higher weight.
+        req.META["HTTP_ACCEPT_LANGUAGE"] = "en-IND;q=0.9,de;q=0.8"
+        self.assertEqual(get_browser_language_code(req), "de")
+
+        # Browser locale is set to unsupported language.
+        req.META["HTTP_ACCEPT_LANGUAGE"] = "en-IND"
+        self.assertIsNone(get_browser_language_code(req))
+
+        req.META["HTTP_ACCEPT_LANGUAGE"] = "*"
+        self.assertIsNone(get_browser_language_code(req))
+
 
 class JsonTranslationTestCase(ZulipTestCase):
     def tearDown(self) -> None: