Laravel middleware that changes to user preferred language (without route prefix).

PreferredLanguage.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Support\Facades\File;
use Illuminate\Support\Str;

class PreferredLanguage
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        if ($request->has('lang') and $this->isSupportedLocale($request->input('lang'))) {
            app()->setLocale($request->input('lang'));
        } else if ($request->session()->has('locale') and $this->isSupportedLocale($request->session()->get('locale'))) {
            app()->setLocale($request->session()->get('locale'));
        } else if ($request->cookies->has('locale') and $this->isSupportedLocale($request->cookie('locale'))) {
            app()->setLocale($request->cookie('locale'));
        } else if ($request->getLanguages()) {
            app()->setLocale($request->getPreferredLanguage($this->getSupportedLanguages()));
        }

        return $next($request);
    }

    /**
     * @param string|null $locale
     * @return bool
     */
    protected function isSupportedLocale($locale): bool
    {
        return in_array($locale, (array) $this->getSupportedLanguages());
    }

    /**
     * @return array|null
     */
    protected function getSupportedLanguages()
    {
        return config('language.support', $this->guessAvailableLanguages());
    }

    /**
     * @return array|null
     */
    protected function guessAvailableLanguages()
    {
        try {
            $lang = array_map(function ($path) {
                return str_replace('-', '_', Str::afterLast($path, DIRECTORY_SEPARATOR));
            }, File::directories(resource_path('lang')));
        } catch (\Symfony\Component\Finder\Exception\DirectoryNotFoundException $exception) {
        } finally {
            return $lang ?? null;
        }
    }
}

in app/Http/Kernel.php

    /**
     * The application's route middleware groups.
     *
     * @var array
     */
    protected $middlewareGroups = [
        'web' => [
            // ...

            \App\Http\Middleware\PreferredLanguage::class,
        ],

        // ...
    ];