app_controllers_locale_controller.rb

class LocaleController < ApplicationController
  respond_to :json
  caches_page :show, :gzip => :best_compression

  def show
    expires_in 1.year
    respond_with YAML::load File.open File.join Rails.root, %W[config locales #{params[:id]}.yml]
  end
end

config_routes.rb

MyApp::Application.routes.draw do
  resources :locale, :only => :show
end