firefly-iii/app/Http/Middleware/Authenticate.php

<?php

namespace FireflyIII\Http\Middleware;

use App;
use Carbon\Carbon;
use Closure;
use Illuminate\Support\Facades\Auth;
use Preferences;

class Authenticate
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request $request
     * @param  \Closure                 $next
     * @param  string|null              $guard
     *
     * @return mixed
     */
    public function handle($request, Closure $next, $guard = null)
    {
        if (Auth::guard($guard)->guest()) {
            if ($request->ajax()) {
                return response('Unauthorized.', 401);
            } else {

                return redirect()->guest('login');
            }
        }

        // if logged in, set user language:
        $pref = Preferences::get('language', env('DEFAULT_LANGUAGE', 'en_US'));
        App::setLocale($pref->data);
        Carbon::setLocale(substr($pref->data, 0, 2));
        $locale = explode(',', trans('config.locale'));
        $locale = array_map('trim', $locale);

        setlocale(LC_TIME, $locale);
        setlocale(LC_MONETARY, $locale);


        return $next($request);
    }
}