Add language dropdown to compose in web UI (#18420)

app/serializers/initial_state_serializer.rb

@@ -2,7 +2,8 @@
 
 class InitialStateSerializer < ActiveModel::Serializer
   attributes :meta, :compose, :accounts,
-             :media_attachments, :settings
+             :media_attachments, :settings,
+             :languages
 
   has_one :push_subscription, serializer: REST::WebPushSubscriptionSerializer
 
@@ -59,6 +60,7 @@ class InitialStateSerializer < ActiveModel::Serializer
       store[:me]                = object.current_account.id.to_s
       store[:default_privacy]   = object.visibility || object.current_account.user.setting_default_privacy
       store[:default_sensitive] = object.current_account.user.setting_default_sensitive
+      store[:default_language]  = object.current_account.user.preferred_posting_language
     end
 
     store[:text] = object.text if object.text
@@ -77,6 +79,10 @@ class InitialStateSerializer < ActiveModel::Serializer
     { accept_content_types: MediaAttachment.supported_file_extensions + MediaAttachment.supported_mime_types }
   end
 
+  def languages
+    LanguagesHelper::SUPPORTED_LOCALES.map { |(key, value)| [key, value[0], value[1]] }
+  end
+
   private
 
   def instance_presenter