diff --git a/docs/ISSUE-226-SHARED-KEYMAP-LOCALE-CONTRACT.md b/docs/ISSUE-226-SHARED-KEYMAP-LOCALE-CONTRACT.md
new file mode 100644
index 0000000..ed305b4
--- /dev/null
+++ b/docs/ISSUE-226-SHARED-KEYMAP-LOCALE-CONTRACT.md
@@ -0,0 +1,30 @@
+# ISSUE-226 — Shared key-map + locale-kontrakt (backend/frontend)
+
+## Source of truth
+- Single shared artifact: `shared/i18n/lobby.json`
+- Ownership is documented under `contract.ownership` in the same artifact.
+
+## Locale contract
+Defined under `contract.locale`:
+- default locale: `en`
+- supported locales: `en`, `da`
+- fallback rule: use default locale when requested locale is unsupported or a key translation is missing.
+
+## Shared backend→frontend key-map
+Defined under `contract.backend_to_frontend_error_keys`.
+
+Examples:
+- `session_not_found -> session_not_found`
+- `session_not_joinable -> join_failed`
+- `round_start_invalid_phase -> start_round_failed`
+
+This allows backend error codes to remain stable while frontend copy keys stay UX-oriented.
+
+## da/en example values
+From `shared/i18n/lobby.json`:
+- `frontend.errors.session_code_required.en = "Session code is required."`
+- `frontend.errors.session_code_required.da = "Sessionskoden er påkrævet."`
+
+## Verification
+- Backend: `python manage.py test lobby.tests.I18nResolverTests`
+- Frontend: `npm test -- --run tests/lobby-i18n.contract.test.ts`
diff --git a/frontend/tests/lobby-i18n.contract.test.ts b/frontend/tests/lobby-i18n.contract.test.ts
index 6137824..c75c6e8 100644
--- a/frontend/tests/lobby-i18n.contract.test.ts
+++ b/frontend/tests/lobby-i18n.contract.test.ts
@@ -14,10 +14,16 @@ describe('shared i18n keyspace contract', () => {
     }
   });
 
-  it('keeps backend error-code keyspace aligned with backend translations', () => {
-    for (const [code, key] of Object.entries(lobbyCatalog.backend.error_codes)) {
-      expect(code).toBe(key);
-      expect(lobbyCatalog.backend.errors[key as keyof typeof lobbyCatalog.backend.errors]).toBeDefined();
+  it('keeps backend error-code keyspace aligned with shared backend→frontend map and backend translations', () => {
+    for (const [code, backendKey] of Object.entries(lobbyCatalog.backend.error_codes)) {
+      const frontendKey =
+        lobbyCatalog.contract.backend_to_frontend_error_keys[
+          code as keyof typeof lobbyCatalog.contract.backend_to_frontend_error_keys
+        ];
+
+      expect(lobbyCatalog.backend.errors[backendKey as keyof typeof lobbyCatalog.backend.errors]).toBeDefined();
+      expect(frontendKey, `missing frontend mapping for ${code}`).toBeTruthy();
+      expect(lobbyCatalog.frontend.errors[frontendKey as keyof typeof lobbyCatalog.frontend.errors]).toBeDefined();
     }
 
     for (const [key, translations] of Object.entries(lobbyCatalog.backend.errors)) {
@@ -42,7 +48,7 @@ describe('lobbyMessage locale handling', () => {
     ).toBe('Sessionskoden er ugyldig, eller sessionen findes ikke længere.');
   });
 
-  it('falls back when backend error code has no frontend translation key', () => {
+  it('uses shared backend→frontend key-map when backend key differs from frontend copy key', () => {
     expect(
       lobbyMessageFromApiPayload(
         { error_code: 'session_not_joinable', locale: 'da' },
@@ -50,4 +56,13 @@ describe('lobbyMessage locale handling', () => {
       ),
     ).toBe('Kunne ikke joine. Tjek kode eller kaldenavn og prøv igen.');
   });
+
+  it('falls back to caller-provided fallback key for unknown backend error codes', () => {
+    expect(
+      lobbyMessageFromApiPayload(
+        { error_code: 'unknown_backend_key', locale: 'da' },
+        'join_failed',
+      ),
+    ).toBe('Kunne ikke joine. Tjek kode eller kaldenavn og prøv igen.');
+  });
 });
diff --git a/lobby/tests.py b/lobby/tests.py
index e3bcb97..15ba351 100644
--- a/lobby/tests.py
+++ b/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 +1273,14 @@ 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.assertIn(key, backend_errors)
+        for code, backend_key in catalog["backend"]["error_codes"].items():
+            frontend_key = shared_map.get(code)
+            self.assertIn(backend_key, backend_errors)
+            self.assertTrue(frontend_key, f"missing frontend mapping for backend code: {code}")
+            self.assertIn(frontend_key, frontend_errors)
diff --git a/shared/i18n/lobby.json b/shared/i18n/lobby.json
index 2fb4e32..9081cff 100644
--- a/shared/i18n/lobby.json
+++ b/shared/i18n/lobby.json
@@ -321,5 +321,31 @@
         "da": "Runden er allerede konfigureret"
       }
     }
+  },
+  "contract": {
+    "ownership": {
+      "artifact": "shared/i18n/lobby.json",
+      "backend": "lobby/* reads backend/errors + backend/error_codes",
+      "frontend": "frontend/* reads frontend/errors + frontend/ui + contract/backend_to_frontend_error_keys"
+    },
+    "locale": {
+      "default": "en",
+      "supported": [
+        "en",
+        "da"
+      ],
+      "fallback": "Use default locale when requested locale is unsupported or key translation is missing."
+    },
+    "backend_to_frontend_error_keys": {
+      "session_code_required": "session_code_required",
+      "nickname_invalid": "nickname_invalid",
+      "session_not_found": "session_not_found",
+      "session_not_joinable": "join_failed",
+      "nickname_taken": "nickname_taken",
+      "category_slug_required": "start_round_failed",
+      "category_not_found": "start_round_failed",
+      "round_start_invalid_phase": "start_round_failed",
+      "round_already_configured": "start_round_failed"
+    }
   }
-}
\ No newline at end of file
+}