3dealer/app/Http/Middleware/SetLocale.php

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;
use Illuminate\Support\Facades\App;
use Illuminate\Support\Facades\URL;
use Illuminate\Support\Facades\View;
use Symfony\Component\HttpFoundation\Response;

class SetLocale
{
    /**
     * Validate the {locale} route segment, apply it app-wide, and share
     * the current/alternate locale (and the alternate's URL) with all views
     * so the header switcher and layout hreflang tags don't recompute it.
     */
    public function handle(Request $request, Closure $next): Response
    {
        $locale = $request->route('locale');
        $available = config('app.available_locales');

        if (! in_array($locale, $available, true)) {
            abort(404);
        }

        App::setLocale($locale);

        // Lets route() calls omit {locale} anywhere in the request lifecycle
        // (controllers, views) — without this, every route() call inside the
        // {locale} group would need locale passed explicitly every time.
        URL::defaults(['locale' => $locale]);

        $altLocale = collect($available)->first(fn ($l) => $l !== $locale);
        $routeName = $request->route()->getName();

        View::share('currentLocale', $locale);
        View::share('altLocale', $altLocale);
        View::share(
            'altLocaleUrl',
            $routeName
                ? route($routeName, array_merge($request->route()->parameters(), ['locale' => $altLocale]))
                : url('/'.$altLocale),
        );

        return $next($request);
    }
}