rails localization setup

application_controller.rb

class ApplicationController < ActionController::Base

  before_filter :set_locale
  # Prevent CSRF attacks by raising an exception.
  # For APIs, you may want to use :null_session instead.
  protect_from_forgery with: :exception

  UNLOCALIZED_NAMESPACES = %w(admin)

  def set_locale
    I18n.locale = params[:locale]
  end

  def default_url_options(options={})
    if is_url_localized?
      _locale = I18n.locale
    else
      _locale = nil
    end
    {locale: _locale}
  end

  def is_url_localized?
    !UNLOCALIZED_NAMESPACES.any? {
      |namespace| params[:controller].include? namespace
    }
  end
end

routes.rb

Rails.application.routes.draw do
  namespace :admin do
    resources :controller
  end
  
  # localized routes
  scope ':locale', locale: /#{I18n.available_locales.join('|')}/ do
    resources :controller
    get 'url', to: 'controller#action', as: action 
  end
  
  get '', to: redirect("/#{I18n.default_locale}")
end