laravelLocalization transRoute（）：如何从url中删除locale前缀

I'm working on a website, which uses french at default language. I'm new on Laravel, and I have to continue the work of somebody else. I want the users to be able to switch the site in english. Everything works fine, unless, the 'en' prefix won't go from the url, even when I switch back to french.

Anyone ever had the same issue ?

Route::group(
    [
        'prefix' => LaravelLocalization::setLocale(),
        'middleware' => [ 'localeSessionRedirect', 'localizationRedirect', 'localize' ]
    ], function ()
    {

All the localized routes are in this Route::group

My controller is :

public function language(String $locale)
{
  \Session::put('lang', $locale);
  return redirect()->back();
}

My middleware is

  public function handle($request, Closure $next)
  {
        if ( \Session::has('lang')) {
            \App::setLocale(\Session::get('lang'));
        }
        return $next($request);
  }