Laravel change locale not working

App::setLocale() is not persistent, and sets locale only for current request(runtime). You can achieve persistent in multiple ways (example of 2):

Route::post('/locale', function(){

     session(['my_locale' => app('request')->input('locale')]);

     return redirect()->back();
});

This will set session key with lang value from request for current user. Next create a Middleware to set locale based on user session language

<?php namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;
use Illuminate\Foundation\Application;

class Language {

    public function __construct(Application $app, Request $request) {
        $this->app = $app;
        $this->request = $request;
    }

    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $this->app->setLocale(session('my_locale', config('app.locale')));

        return $next($request);
    }

}

This will get current session and if is empty will fallback to default locale, which is set in your app config.

In app\Http\Kernel.php add previously created Language middleware:

protected $middleware = [
   \App\Http\Middleware\Language::class,
];

As global middlware or just for web (based on your needs).

Scenario №2 – Lang based on URL path
Create an array with all available locales on your app inside app config

'available_locale' => ['fr', 'gr', 'ja'],

Inside the Middleware we will check the URL first segment en, fr, gr, cy if this segment is in available_locale, set language

public function handle($request, Closure $next)
{
      if(in_array($request->segment(1), config('app.available_locale'))){
            $this->app->setLocale($request->segment(1));
      }else{
            $this->app->setLocale(config('app.locale'));
      }

      return $next($request);
}

You will need to modify app\Providers\RouteServiceProvider for setting prefix to all your routes. so you can access them example.com or example.com/fr/ with French language
Find: mapWebRoutes
And add this to it: (before add use Illuminate\Http\Request;)

public function map(Request $request)
    {
        $this->mapApiRoutes();
        $this->mapWebRoutes($request);
    }
    protected function mapWebRoutes(Request $request)
    {
        $locale = null;
        if(in_array($request->segment(1), config('app.available_locale'))){
          $locale = $request->segment(1);
        }

        Route::group([
           'middleware' => 'web',
           'namespace' => $this->namespace,
           'prefix' => $locale
        ], function ($router) {
             require base_path('routes/web.php');
        });
    }

This will prefix all your routes with country letter like ‘fr gr cy’ except en for non-duplicate content, so is better to not add into available_locales_array