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

Thank you Daniel Samuels for test project.

2 files changed, 4 insertions, 3 deletions

diff --git a/django/utils/translation/trans_null.py b/django/utils/translation/trans_null.py
index a687572b69..c8bfb1256e 100644
--- a/django/utils/translation/trans_null.py
+++ b/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)
diff --git a/django/utils/translation/trans_real.py b/django/utils/translation/trans_real.py
index 7c81e8bee2..423f30eaba 100644
--- a/django/utils/translation/trans_real.py
+++ b/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.