NotFoundExeception in RouteCollection when using multiple route groups

I define in RouteServiceProvider.php this map implementation:

public function map(Router $router, Request $request)
{
    $locale = $request->segment(1);
    $this->app->setLocale($locale);

    $router->group(['prefix' => '{lang}'], function($router) {
        require app_path('Http/routes.php');
    });
}  

In app/routes.php Ι also define a route group

Route::group(['prefix' => 'admin', 'middleware' => 'auth'], function (){

//code

);

Now when Ι try to access to admin/xxx it show me this exception

NotFoundHttpException in RouteCollection.php line 161:

I try to put the definition of the first route group in routes.php but that didn't work.

How can Ι solve the problem?

via Chebli Mohamed