Laravel: optional language route prefix (e.g. domain.com/fr/about or without prefix domain.com/about)

GetRoutes.php

// include any routes you need here that are accessed with GET method
// as POST routes don't need prefixing usually
Route::get('/', 'HomeController@index');
Route::get('/about', 'HomeController@showAbout');

helpers.php

<?php
// custom loc_url helpers for using in blade template
if (! function_exists('loc_url')) {
    /**
     * Return URL with lang prefix to slug
     */
    function loc_url($slug)
    {
        $locale = app()->getLocale();

        if($slug === '/') {
            return $locale === 'en' ? secure_url('/') : secure_url($locale);
        }
        // if there is no language prefix, return url without prefix
        return $locale === 'en' ? secure_url($slug) : secure_url($locale . '/' . $slug);
    }
}

Locale.php

<?php
// Locale.php middleware
namespace App\Http\Middleware;

use Closure;

class Locale
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        if ($request->method() === 'GET') {
            $locale = config('app.locale');
            $segment = $request->segment(1);

            if (in_array($segment, config('app.locales'))) {
                $locale = $segment;
            }

            session(['locale' => $locale]);
            app()->setLocale($locale);
        }

        return $next($request);
    }
}

web.php

// a group with empty prefix
Route::group([
    'middleware' => 'locale',
], function() {
    require base_path().'/routes/GetRoutes.php';
});

// a group with lang prefix
Route::group([
    'prefix' => '{lang?}',
    'middleware' => 'locale',
], function() {
    require base_path().'/routes/GetRoutes.php';
});

I had problems placing optional route prefix in my Laravel app. I found this solution and decided to share that.

where are the files located?