diff --git a/lobby/tests.py b/lobby/tests.py
index 6b156f3..1aefac6 100644
--- a/lobby/tests.py
+++ b/lobby/tests.py
@@ -360,6 +360,8 @@ class LieSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "lie_submission_closed")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Lie submission window has closed")
 
     def test_submit_lie_rejects_duplicate_submission(self):
@@ -381,6 +383,8 @@ class LieSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 409)
+        self.assertEqual(response.json()["error_code"], "lie_already_submitted")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Lie already submitted for this player")
 
     def test_submit_lie_requires_session_token(self):
@@ -401,6 +405,8 @@ class LieSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "session_token_required")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "session_token is required")
 
     def test_submit_lie_rejects_invalid_session_token(self):
@@ -421,8 +427,33 @@ class LieSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.json()["error_code"], "invalid_player_session_token")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Invalid player session token")
 
+    def test_submit_lie_uses_danish_locale_payload_from_accept_language(self):
+        round_question = RoundQuestion.objects.create(
+            session=self.session,
+            round_number=1,
+            question=self.question,
+            correct_answer=self.question.correct_answer,
+        )
+
+        response = self.client.post(
+            reverse(
+                "lobby:submit_lie",
+                kwargs={"code": self.session.code, "round_question_id": round_question.id},
+            ),
+            data={"player_id": self.player.id, "session_token": "invalid-token", "text": "Sydney"},
+            content_type="application/json",
+            HTTP_ACCEPT_LANGUAGE="da-DK,da;q=0.9,en;q=0.1",
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.json()["error_code"], "invalid_player_session_token")
+        self.assertEqual(response.json()["locale"], "da")
+        self.assertEqual(response.json()["error"], "Ugyldigt spiller-session-token")
+
 class MixAnswersTests(TestCase):
     def setUp(self):
         self.host = User.objects.create_user(username="host", password="secret123")
@@ -582,6 +613,8 @@ class GuessSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "guess_submission_invalid_phase")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Guess submission is only allowed in guess phase")
 
     def test_submit_guess_rejects_unknown_answer(self):
@@ -595,6 +628,8 @@ class GuessSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "selected_answer_invalid")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Selected answer is not part of this round")
 
     def test_submit_guess_rejects_duplicate_submission(self):
@@ -610,6 +645,8 @@ class GuessSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 409)
+        self.assertEqual(response.json()["error_code"], "guess_already_submitted")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Guess already submitted for this player")
 
     def test_submit_guess_rejects_after_deadline(self):
@@ -626,6 +663,8 @@ class GuessSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "guess_submission_closed")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Guess submission window has closed")
 
 
@@ -641,6 +680,8 @@ class GuessSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "session_token_required")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "session_token is required")
 
     def test_submit_guess_rejects_invalid_session_token(self):
@@ -654,6 +695,8 @@ class GuessSubmissionTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.json()["error_code"], "invalid_player_session_token")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Invalid player session token")
 
 
@@ -735,6 +778,8 @@ class ScoreCalculationTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.json()["error_code"], "host_only_calculate_scores")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Only host can calculate scores")
 
     def test_calculate_scores_rejects_duplicate_calculation(self):
@@ -756,6 +801,8 @@ class ScoreCalculationTests(TestCase):
 
         self.assertEqual(first.status_code, 200)
         self.assertEqual(second.status_code, 409)
+        self.assertEqual(second.json()["error_code"], "scores_already_calculated")
+        self.assertEqual(second.json()["locale"], "en")
         self.assertEqual(second.json()["error"], "Scores already calculated for this round question")
 
 
@@ -793,6 +840,8 @@ class RevealRoundFlowTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.json()["error_code"], "host_only_view_scoreboard")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Only host can view scoreboard")
 
     def test_host_can_finish_game_from_reveal(self):
@@ -825,6 +874,8 @@ class RevealRoundFlowTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.json()["error_code"], "host_only_finish_game")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Only host can finish game")
 
     def test_finish_game_rejects_wrong_phase(self):
@@ -840,6 +891,8 @@ class RevealRoundFlowTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "finish_game_invalid_phase")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Game can only be finished from reveal phase")
 
     def test_host_can_start_next_round_from_reveal(self):
@@ -874,8 +927,26 @@ class RevealRoundFlowTests(TestCase):
         )
 
         self.assertEqual(response.status_code, 400)
+        self.assertEqual(response.json()["error_code"], "next_round_invalid_phase")
+        self.assertEqual(response.json()["locale"], "en")
         self.assertEqual(response.json()["error"], "Next round can only start from reveal phase")
 
+    def test_reveal_scoreboard_unsupported_locale_falls_back_to_en_deterministically(self):
+        self.client.login(username="other_reveal", password="secret123")
+
+        response = self.client.get(
+            reverse(
+                "lobby:reveal_scoreboard",
+                kwargs={"code": self.session.code},
+            ),
+            HTTP_ACCEPT_LANGUAGE="fr-FR,fr;q=0.9",
+        )
+
+        self.assertEqual(response.status_code, 403)
+        self.assertEqual(response.json()["error_code"], "host_only_view_scoreboard")
+        self.assertEqual(response.json()["locale"], "en")
+        self.assertEqual(response.json()["error"], "Only host can view scoreboard")
+
 class UiScreenTests(TestCase):
     def setUp(self):
         self.host = User.objects.create_user(username="host_ui", password="secret123")
@@ -1342,6 +1413,15 @@ class I18nResolverTests(TestCase):
         self.assertEqual(result, "missing_key")
         self.assertTrue(any("i18n key missing in shared catalog" in entry for entry in logs.output))
 
+    def test_missing_backend_error_code_is_logged_with_context(self):
+        from lobby.i18n import resolve_error_key
+
+        with self.assertLogs("lobby.i18n", level="WARNING") as logs:
+            result = resolve_error_key("missing_code")
+
+        self.assertEqual(result, "missing_code")
+        self.assertTrue(any("i18n error code missing in shared catalog" in entry for entry in logs.output))
+
     def test_missing_locale_translation_falls_back_to_default_locale(self):
         with patch(
             "lobby.i18n.lobby_i18n_error_messages",