Add ability to filter followed accounts' posts by language (#19095)

app/serializers/rest/relationship_serializer.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class REST::RelationshipSerializer < ActiveModel::Serializer
-  attributes :id, :following, :showing_reblogs, :notifying, :followed_by,
+  attributes :id, :following, :showing_reblogs, :notifying, :languages, :followed_by,
              :blocking, :blocked_by, :muting, :muting_notifications, :requested,
              :domain_blocking, :endorsed, :note
 
@@ -25,6 +25,11 @@ class REST::RelationshipSerializer < ActiveModel::Serializer
       false
   end
 
+  def languages
+    (instance_options[:relationships].following[object.id] || {})[:languages] ||
+      (instance_options[:relationships].requested[object.id] || {})[:languages]
+  end
+
   def followed_by
     instance_options[:relationships].followed_by[object.id] || false
   end