LocaleListener.php

<?php
namespace Acme\Bundle\I18nBundle\EventListener;
use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
class LocaleListener
{
    private $container;
    private $locales;
    public function __construct(ContainerInterface $container, array $locales)
    {
        $this->container = $container;
        $this->locales = $locales;
    }
    public function onKernelRequest(GetResponseEvent $event)
    {
        if (HttpKernelInterface::MASTER_REQUEST !== $event->getRequestType()) {
            return;
        }
        $request = $event->getRequest();
        $locale = $request->getPreferredLanguage($this->locales);
        if($this->container->has('session')) {
            $session = $this->container->get('session');
            if($session->get('_locale', false)) {
                $request->setLocale($session->get('_locale'));
                return;
            }
            $session->set('_locale', $locale);
        }
        $request->setLocale($locale);
    }
}

services.yml

parameters:
    acme_i18n.listener.locale_listener.class: Acme\Bundle\I18nBundle\EventListener\LocaleListener

services:
    acme_i18n.listener.locale_listener:
        class: %acme_i18n.listener.locale_listener.class%
        arguments: [ @service_container, "%locales%" ]
        tags:
            - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest, priority: -255 }