Merge branch 'main' into glitch-soc/merge-upstream

Conflicts:
- `app/javascript/mastodon/features/compose/components/compose_form.js`:
  Conflict caused because of minor code style change upstream, while glitch-soc
  has different code for handling variable maximum chars.
  Ported the change.
- `app/serializers/initial_state_serializer.rb`:
  Conflict because glitch-soc had two extra attributes where upstream added
  one.
  Added upstream's attribute.

app/models/user.rb

@@ -53,6 +53,7 @@ class User < ApplicationRecord
   include Settings::Extend
   include UserRoles
   include Redisable
+  include LanguagesHelper
 
   # The home and list feeds will be stored in Redis for this amount
   # of time, and status fan-out to followers will include only people
@@ -248,7 +249,7 @@ class User < ApplicationRecord
   end
 
   def preferred_posting_language
-    settings.default_language || locale
+    valid_locale_cascade(settings.default_language, locale)
   end
 
   def setting_default_privacy