How to use a dynamic variable in Laravel group routes?

for example:

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

create a variable in middleware:

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

$lang = session('locale');

App::setLocale($lang);

return $next($request);      
});

also tried to get the data in the prefix, but got null

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

or

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

change language separate route through session

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

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

return redirect()->back();

})->name('setlocale');

Thank you in advance for your help.

via Chebli Mohamed