BookStack/app/Http/Middleware/Localization.php

<?php

namespace BookStack\Http\Middleware;

use Carbon\Carbon;
use Closure;

class Localization
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $defaultLang = config('app.locale');
        $locale = setting()->getUser(user(), 'language', $defaultLang);
        app()->setLocale($locale);
        Carbon::setLocale($locale);
        return $next($request);
    }
}