fix(issue-225): honor Accept-Language fallback chain in locale resolver

lobby/i18n.py

@@ -37,10 +37,14 @@ def lobby_i18n_error_messages() -> dict:
 def resolve_locale(request: HttpRequest) -> str:
     default_locale, supported_locales = i18n_locale_config()
 
-    raw_accept_language = (request.META.get("HTTP_ACCEPT_LANGUAGE") or "").split(",", 1)[0]
-    raw_requested = raw_accept_language.split(";", 1)[0].strip().replace("_", "-").split("-", 1)[0].lower()
-    if raw_requested in supported_locales:
-        return raw_requested
+    accept_language = request.META.get("HTTP_ACCEPT_LANGUAGE") or ""
+    for candidate in accept_language.split(","):
+        tag, _sep, quality = candidate.partition(";")
+        if "q=0" in quality.replace(" ", ""):
+            continue
+        normalized = tag.strip().replace("_", "-").split("-", 1)[0].lower()
+        if normalized in supported_locales:
+            return normalized
 
     requested = (get_language_from_request(request) or "").replace("_", "-").split("-", 1)[0].lower()
     if requested in supported_locales:

lobby/tests.py

@@ -1274,6 +1274,28 @@ class I18nResolverTests(TestCase):
         self.assertEqual(response.status_code, 400)
         self.assertEqual(resolve_locale(response.wsgi_request), "da")
 
+    def test_resolve_locale_uses_next_supported_language_when_primary_is_unsupported(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "", "nickname": "Luna"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="fr-FR, da;q=0.9, en;q=0.8",
+        )
+
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(resolve_locale(response.wsgi_request), "da")
+
+    def test_resolve_locale_skips_q0_languages_and_falls_back_to_next_supported(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "", "nickname": "Luna"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="da;q=0, en;q=0.8",
+        )
+
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(resolve_locale(response.wsgi_request), "en")
+
     def test_resolve_locale_defaults_to_en_when_header_missing(self):
         response = self.client.post(
             reverse("lobby:join_session"),