application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  force_ssl if: :ssl_configured?
  before_action :set_locale

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

  def default_url_options
    { locale: I18n.locale == I18n.default_locale ? nil : I18n.locale  }
  end
  
  def ssl_configured?
    !Rails.env.development?
  end
end

routes.rb

Rails.application.routes.draw do
  scope "/:locale" do
    resources :messages
    resource :session
    root 'static#home'
    get 'about', to: 'static#about'
    get 'imprint', to: 'static#imprint'
  end

end