Add contact_account and languages to instance API (#6574)

app/serializers/rest/instance_serializer.rb

@@ -4,7 +4,12 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :uri, :title, :description, :email,
-             :version, :urls, :stats, :thumbnail
+             :version, :urls, :stats, :thumbnail,
+             :languages
+
+  has_one :contact_account, serializer: REST::AccountSerializer
+
+  delegate :contact_account, to: :instance_presenter
 
   def uri
     Rails.configuration.x.local_domain
@@ -42,6 +47,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     { streaming_api: Rails.configuration.x.streaming_api_base_url }
   end
 
+  def languages
+    [ENV.fetch('DEFAULT_LOCALE', I18n.default_locale)]
+  end
+
   private
 
   def instance_presenter