Fix preferred posting language returning unusable value in REST API (#18428)

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

app/serializers/rest/preferences_serializer.rb

@@ -17,7 +17,7 @@ class REST::PreferencesSerializer < ActiveModel::Serializer
   end
 
   def posting_default_language
-    object.user.setting_default_language.presence
+    object.user.preferred_posting_language
   end
 
   def reading_default_sensitive_media