Route::group(['prefix' => '{lang?}'], function () {
//..
});

public function handle($request, Closure $next){

$lang = session('locale');

return $next($request);
});

Route::group(['prefix' =>  config('app.locale')], function () {
//..
});

Route::group(['prefix' =>  session('locale')], function () {
//..
});

Route::get('setlocale/{locale}', function ($locale) {

session(['locale' => $locale]);

return redirect()->back();

})->name('setlocale');