Set current locale

app/controllers/application_controller.rb:

around_action :switch_locale

def switch_locale(&action)
  locale = params[:locale] || I18n.default_locale
  I18n.with_locale(locale, &action)
end

def default_url_options
  { locale: I18n.locale }
end

config/routes.rb:

scope "/:locale" do
  resources :cats
end