Certified/app/Http/Controllers/LanguageController.php

<?php

namespace App\Http\Controllers;

use App\Enums\Locale;
use Illuminate\Http\RedirectResponse;
use Illuminate\Http\Request;

class LanguageController extends Controller
{
    /**
     * Switch the active language. Persisted to the user's account when
     * authenticated, otherwise kept in the session for the duration of the
     * guest visit (e.g. while browsing the login/register pages).
     */
    public function __invoke(Request $request, string $locale): RedirectResponse
    {
        abort_unless(in_array($locale, Locale::values(), true), 404);

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

        if ($request->user()) {
            $request->user()->locale = $locale;
            $request->user()->save();
        }

        return back();
    }
}