✨(backend) add API access for 'language' attribute on User model

- allow the language attribute on the user to be updated via API
- add frontend function to update the user language via API
- extend defaults on the test users, to have fixed language in E2E tests
- extend types and variables using the types with the new field

src/backend/demo/defaults.py

@@ -7,17 +7,12 @@ NB_OBJECTS = {
 }
 
 DEV_USERS = [
+    {"username": "impress", "email": "impress@impress.world", "language": "en-us"},
+    {"username": "user-e2e-webkit", "email": "user@webkit.e2e", "language": "en-us"},
+    {"username": "user-e2e-firefox", "email": "user@firefox.e2e", "language": "en-us"},
     {
-        "username": "impress",
-        "email": "impress@impress.world",
+        "username": "user-e2e-chromium",
+        "email": "user@chromium.e2e",
+        "language": "en-us",
     },
-    {
-        "username": "user-e2e-webkit",
-        "email": "user@webkit.e2e",
-    },
-    {
-        "username": "user-e2e-firefox",
-        "email": "user@firefox.e2e",
-    },
-    {"username": "user-e2e-chromium", "email": "user@chromium.e2e"},
 ]

src/backend/demo/management/commands/create_demo.py

@@ -179,7 +179,8 @@ def create_demo(stdout):
                     is_superuser=False,
                     is_active=True,
                     is_staff=False,
-                    language=random.choice(settings.LANGUAGES)[0],
+                    language=dev_user["language"]
+                    or random.choice(settings.LANGUAGES)[0],
                 )
             )