Why session expire in next request ?

i try many time and my Language session still expire when i goto next page this is my Code

middleware:

 public function handle($request, Closure $next)
    {
        
        if (App::isLocale('en'))
        {
            $request->session()->put('lang', '');
        } 
        elseif (App::isLocale('kh'))
        {
            $request->session()->put('lang', '_kh');
        }
        
        return $next($request);
    }

my baseController so i call isLanguageSwitcher in baseController __construct() so other Controller that extent from baseController will have Lang session

public function isLanguageSwitcher()
    {
    if(Request::has('kh_lang'))
    {
          App::setLocale('kh');
           // return redirect()->back();
    }
    elseif (Request::has('en_lang'))
    {
            App::setLocale('en');
           // return redirect()->back();
    }
}

my Blade language form

<form method="post">
      {{ csrf_field() }}
      {!!
        (App::isLocale('en') ) 
        ? '<button type="submit" name="kh_lang">Khmer</button>'
        : '<button type="submit" name="en_lang">English</button>'
      !!}
      
    </form>

language is change when i click English button but when i goto next page Session('lang') expire and return to default value kh i'm new to laravel not sure what wrong with my logic.

1. how to solve session expire in next request ?

2. other question is how to prevent F5(refresh page) that pop resubmit form ? coz i try redirect->back() it still pop resubmit.

Thank in advance. :D