test(lobby): cover backend locale resolver normalization and default fallback

lobby/tests.py

@@ -20,7 +20,7 @@ from fupogfakta.models import (
     RoundConfig,
     RoundQuestion,
 )
-from lobby.i18n import i18n_locale_config, lobby_i18n_catalog, resolve_error_message
+from lobby.i18n import i18n_locale_config, lobby_i18n_catalog, resolve_error_message, resolve_locale
 
 User = get_user_model()
 
@@ -1216,6 +1216,27 @@ class SmokeStagingCommandTests(TestCase):
 
 
 class I18nResolverTests(TestCase):
+    def test_resolve_locale_accepts_language_tags_and_normalizes_to_supported_base_locale(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "", "nickname": "Luna"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="da-DK,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_defaults_to_en_when_header_missing(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "", "nickname": "Luna"},
+            content_type="application/json",
+        )
+
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(resolve_locale(response.wsgi_request), "en")
+
     def test_missing_backend_key_returns_key_deterministically(self):
         self.assertEqual(resolve_error_message(key="missing_key", locale="da"), "missing_key")
 
@@ -1252,10 +1273,13 @@ class I18nResolverTests(TestCase):
             self.assertTrue(translations.get("en"), f"frontend key {key} missing en")
             self.assertTrue(translations.get("da"), f"frontend key {key} missing da")
 
-    def test_backend_error_codes_map_to_same_named_translation_keys(self):
+    def test_backend_error_codes_map_via_shared_backend_frontend_key_map(self):
         catalog = lobby_i18n_catalog()
         backend_errors = catalog["backend"]["errors"]
+        frontend_errors = catalog["frontend"]["errors"]
+        shared_map = catalog["contract"]["backend_to_frontend_error_keys"]
 
         for code, key in catalog["backend"]["error_codes"].items():
-            self.assertEqual(code, key)
+            self.assertEqual(shared_map.get(code), key)
             self.assertIn(key, backend_errors)
+            self.assertIn(key, frontend_errors)