users: Use browser locale to set the default language of new user.

This commit reads the browser locale during user registration, and
sets it as default language of user if it is supported by Zulip.
Otherwise, it is set to realm's default language.

zerver/lib/create_user.py

@@ -84,6 +84,7 @@ def create_user_profile(
     is_mirror_dummy: bool,
     tos_version: Optional[str],
     timezone: Optional[str],
+    default_language: str = "en",
     tutorial_status: str = UserProfile.TUTORIAL_WAITING,
     enter_sends: bool = False,
     force_id: Optional[int] = None,
@@ -116,7 +117,7 @@ def create_user_profile(
         tutorial_status=tutorial_status,
         enter_sends=enter_sends,
         onboarding_steps=orjson.dumps([]).decode(),
-        default_language=realm.default_language,
+        default_language=default_language,
         delivery_email=email,
         **extra_kwargs,
     )
@@ -143,6 +144,7 @@ def create_user(
     timezone: str = "",
     avatar_source: str = UserProfile.AVATAR_FROM_GRAVATAR,
     is_mirror_dummy: bool = False,
+    default_language: str = "en",
     default_sending_stream: Optional[Stream] = None,
     default_events_register_stream: Optional[Stream] = None,
     default_all_public_streams: Optional[bool] = None,
@@ -162,6 +164,7 @@ def create_user(
         is_mirror_dummy,
         tos_version,
         timezone,
+        default_language,
         force_id=force_id,
         force_date_joined=force_date_joined,
     )