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/controllers/api/v1/instances/languages_controller.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class Api::V1::Instances::LanguagesController < Api::BaseController
+  skip_before_action :require_authenticated_user!, unless: :limited_federation_mode?
+  skip_around_action :set_locale
+
+  before_action :set_languages
+
+  vary_by ''
+
+  def show
+    cache_even_if_authenticated!
+    render json: @languages, each_serializer: REST::LanguageSerializer
+  end
+
+  private
+
+  def set_languages
+    @languages = LanguagesHelper::SUPPORTED_LOCALES.keys.map { |code| LanguagePresenter.new(code) }
+  end
+end