Symfony Locale Listener that redirects to default language route if no language has been added instead of 404

LocaleListener.php

<?php
namespace YourBundle\EventListener;

use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\Routing\RouterInterface;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Routing\Matcher\UrlMatcher;
use Symfony\Component\Routing\RequestContext;

class LocaleListener implements EventSubscriberInterface
{

    /**
    * @var routeCollection \Symfony\Component\Routing\RouteCollection
    */
    private $routeCollection;

    /**
    * @var urlMatcher \Symfony\Component\Routing\Matcher\UrlMatcher;
    */
    private $urlMatcher;


    private $oldUrl;
    private $newUrl;
    private $languages;
    private $defaultLanguage;
    
    public function __construct(RouterInterface $router, $languages, $defaultLanguage = 'de')
    {
        $this->routeCollection = $router->getRouteCollection();
        $this->languages = $languages;
        $this->defaultLanguage = $defaultLanguage;
        $context = new RequestContext("/");
    }

    public function onKernelRequest(GetResponseEvent $event)
    {
        //GOAL:
        // Redirect all incoming requests to their /locale/route equivalent when exists.
        // Do nothing if it already has /locale/ in the route to prevent redirect loops
        // Do nothing if the route requested has no locale param

        $request = $event->getRequest();
        $this->newUrl  = $request->getPathInfo();
        $this->oldUrl = $request->headers->get('referer');
        $locale = $this->checkLanguage();
        if($locale === null) return;
        
        $request->setLocale($locale);

        $pathLocale = "/".$locale.$this->newUrl;

        //We have to catch the ResourceNotFoundException
        try {
            //Try to match the path with the local prefix
            $this->urlMatcher->match($pathLocale);
            $event->setResponse(new RedirectResponse($pathLocale));
        } catch (\Symfony\Component\Routing\Exception\ResourceNotFoundException $e) {

        } catch (\Symfony\Component\Routing\Exception\MethodNotAllowedException $e) {

        }
    }
    private function checkLanguage(){ 
        foreach($this->languages as $language){
            if(preg_match_all("/\/$language\//", $this->newUrl))
                return null;   
            if(preg_match_all("/\/$language\//", $this->oldUrl))
                return $language;
        }

        return $this->defaultLanguage;
    }
    public static function getSubscribedEvents()
    {
        return array(
            // must be registered before the default Locale listener
            KernelEvents::REQUEST => array(array('onKernelRequest', 17)),
        );
    }
}

routing.yml

language_routing:
    resource: "@YourBundle/Resources/config/multilanguage-routes.yml"
    prefix:   /{_locale}
    requirements:
        _locale: de|en
    defaults: { _locale: de}

services.yml

parameters:
  locale: de
  languages: [de, en]

services:
    app.locale.listener:
       class: YourBundle\EventListener\LocaleListener
       arguments: ["@router","%languages%","%locale%"]
       tags:
           - { name: kernel.event_subscriber }

Fixed, didn't use this code in a long time, it can be written a lot better. If I find the time I will refactor all gists.

@kunicmarko20, instead of redirecting, you can have optional locale prefix in your URL.

For example you can have:

http://example.com/
http://example.com/en/
http://example.com/pl/

or

http://example.com/text/
http://example.com/en/test/
http://example.com/pl/test/

The trick is to configure controllers to have prefix and set one of prefixes as a forward slash (/):

### config/routes/annotations.yaml

controllers:
    resource: ../../src/Controller/
    type: annotation
    prefix:
        /: '/'
        en: '/en'
        pl: '/pl'
    locale: ''
    defaults:
        _locale: ''
    requirements:
        _locale: 'en|pl'
    trailing_slash_on_root: true
kernel:
    resource: ../../src/Kernel.php
    type: annotation

Notice that request attribute parameter _locale will be "/" when you enter path without giving locale.

Then I handle my locale through subscriber like so:

<?php
declare(strict_types=1);

namespace App\EventSubscriber;

use App\Entity\Config\ConfigApp;
use App\Entity\User;
use App\Http\Requests\Request;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Component\HttpKernel\Event\RequestEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\Security\Core\Security;

/**
 * @final
 */
class LocaleSubscriber implements EventSubscriberInterface
{
    /**
     * @var ConfigApp
     */
    private $configApp;

    /**
     * @var Security
     */
    private $security;

    /**
     * @param ConfigApp $configApp
     * @param Security $security
     */
    public function __construct(ConfigApp $configApp, Security $security)
    {
        $this->configApp = $configApp;
        $this->security = $security;
    }

    /**
     * {@inheritDoc}
     */
    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::REQUEST => [['onKernelRequest', 21]],
        ];
    }

    /**
     * @param RequestEvent $event
     */
    public function onKernelRequest(RequestEvent $event)
    {
        /** @var Request $request */
        $request = $event->getRequest();

        /** @var Session $session */
        $session = $request->getSession();

        /** @var string $defaultLocale */
        $defaultLocale = $this->configApp->getLocale();

        /** @var User|null $user */
        $user = $this->security->getUser();

        if ($user && false === empty($user->getLocale())) {
            $defaultLocale = $user->getLocale();
        }

        try {
            $locale = $request->attributes->get('_locale');

            if ('/' === $locale) {
                $locale = $session->get('_locale', $defaultLocale);
            } else {

            }
            
            $session->set('_locale', $locale);
            $request->attributes->set('_locale', $locale);
            $request->setLocale($locale);
        } catch (\Error $e) {

        } catch (\Exception $e) {

        }
    }
}