Fixed #23689 -- Made parsing HTTP Accept-Language header case-insensitive.

Thank you Daniel Samuels for test project.

django/utils/translation/trans_null.py

@@ -61,7 +61,7 @@ def get_language_from_path(request):
 
 
 def get_supported_language_variant(lang_code, strict=False):
-    if lang_code == settings.LANGUAGE_CODE:
+    if lang_code and lang_code.lower() == settings.LANGUAGE_CODE.lower():
         return lang_code
     else:
         raise LookupError(lang_code)

django/utils/translation/trans_real.py

@@ -478,8 +478,9 @@ def check_for_language(lang_code):
 def get_languages():
     """
     Cache of settings.LANGUAGES in a dictionary for easy lookups by key.
+    Convert keys to lowercase as they should be treated as case-insensitive.
     """
-    return dict(settings.LANGUAGES)
+    return {key.lower(): value for key, value in dict(settings.LANGUAGES).items()}
 
 
 @functools.lru_cache(maxsize=1000)
@@ -510,7 +511,7 @@ def get_supported_language_variant(lang_code, strict=False):
         supported_lang_codes = get_languages()
 
         for code in possible_lang_codes:
-            if code in supported_lang_codes and check_for_language(code):
+            if code.lower() in supported_lang_codes and check_for_language(code):
                 return code
         if not strict:
             # if fr-fr is not supported, try fr-ca.

tests/i18n/tests.py

@@ -1902,9 +1902,10 @@ class MiscTests(SimpleTestCase):
         USE_I18N=True,
         LANGUAGES=[
             ("en", "English"),
+            ("ar-dz", "Algerian Arabic"),
             ("de", "German"),
             ("de-at", "Austrian German"),
-            ("pt-br", "Portuguese (Brazil)"),
+            ("pt-BR", "Portuguese (Brazil)"),
         ],
     )
     def test_get_supported_language_variant_real(self):
@@ -1915,8 +1916,11 @@ class MiscTests(SimpleTestCase):
         self.assertEqual(g("de-at"), "de-at")
         self.assertEqual(g("de-ch"), "de")
         self.assertEqual(g("pt-br"), "pt-br")
+        self.assertEqual(g("pt-BR"), "pt-BR")
         self.assertEqual(g("pt"), "pt-br")
         self.assertEqual(g("pt-pt"), "pt-br")
+        self.assertEqual(g("ar-dz"), "ar-dz")
+        self.assertEqual(g("ar-DZ"), "ar-DZ")
         with self.assertRaises(LookupError):
             g("pt", strict=True)
         with self.assertRaises(LookupError):
@@ -1946,7 +1950,6 @@ class MiscTests(SimpleTestCase):
         LANGUAGES=[
             ("en", "English"),
             ("en-latn-us", "Latin English"),
-            ("en-Latn-US", "BCP 47 case format"),
             ("de", "German"),
             ("de-1996", "German, orthography of 1996"),
             ("de-at", "Austrian German"),
@@ -1970,6 +1973,7 @@ class MiscTests(SimpleTestCase):
             ("/de/", "de"),
             ("/de-1996/", "de-1996"),
             ("/de-at/", "de-at"),
+            ("/de-AT/", "de-AT"),
             ("/de-ch/", "de"),
             ("/de-ch-1901/", "de-ch-1901"),
             ("/de-simple-page-test/", None),