protected $languages = ['en', 'ar'];

public function handle($request, Closure $next)
{
    Session::has('applocale') ? Session::get('applocale') : Session::set('applocale', 'en');

    if (in_array(Session::get('applocale'), $this->languages)) {
        $lang = Session::get('applocale');
        Session::set('applocale', $lang);
        App::setLocale($lang);
    } else {
        Session::set('applocale', 'en');
        App::setLocale('en');
    }

    return $next($request);
}

Route::group(['middleware' => ['web', 'language']], function () {
    // app routes
});

Route::group(['middleware' => ['web', 'language']], function () {
    Route::group(['prefix' => '{lang}'], function () {
         // app routes
    });
});