Add GET /api/v1/instance/languages to REST API (#24443)

Co-authored-by: Eugen Rochko <eugen@zeonfederated.com>
Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/serializers/rest/language_serializer.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class REST::LanguageSerializer < ActiveModel::Serializer
+  attributes :code, :name
+end