feat(i18n): add da/en locale pipeline and shared backend key resolver

lobby/tests.py

@@ -19,6 +19,7 @@ from fupogfakta.models import (
     RoundConfig,
     RoundQuestion,
 )
+from lobby.i18n import resolve_error_message
 
 User = get_user_model()
 
@@ -110,6 +111,32 @@ class LobbyFlowTests(TestCase):
         self.assertEqual(response.status_code, 400)
         self.assertEqual(response.json()["error"], "Session is not joinable")
 
+    def test_join_error_localizes_to_danish_with_accept_language_header(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "   ", "nickname": "Luna"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="da",
+        )
+
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "session_code_required")
+        self.assertEqual(response.json()["locale"], "da")
+        self.assertEqual(response.json()["error"], "Sessionskode er påkrævet")
+
+    def test_join_error_falls_back_to_english_for_unsupported_locale(self):
+        response = self.client.post(
+            reverse("lobby:join_session"),
+            data={"code": "   ", "nickname": "Luna"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="fr",
+        )
+
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "session_code_required")
+        self.assertEqual(response.json()["locale"], "en")
+        self.assertEqual(response.json()["error"], "Session code is required")
+
     def test_session_detail_returns_players(self):
         session = GameSession.objects.create(host=self.host, code="LMNO45")
         Player.objects.create(session=session, nickname="Mia", score=7)
@@ -1174,3 +1201,8 @@ class SmokeStagingCommandTests(TestCase):
                     "finish_game",
                 ],
             )
+
+
+class I18nResolverTests(TestCase):
+    def test_missing_backend_key_returns_key_deterministically(self):
+        self.assertEqual(resolve_error_message(key="missing_key", locale="da"), "missing_key")