confiy.yml

parameters:
  locales: [en, nl]

Listener.php

<?php

namespace ACME\ExampleBundle\Listener;

use Gedmo\Translatable\TranslatableListener as Listener;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * TranslationListener
 */
class TranslationListener
{
    protected $listener;
    protected $container;
    
    public function __construct(Listener $listener, ContainerInterface $container)
    {
        $this->listener = $listener;
        $this->container = $container;
    }

    /**
     * Set the translation listener locale from the session.
     *
     * This method should be attached to the kernel.request event.
     *
     * @param GetResponseEvent $event
     */
    public function onKernelRequest(GetResponseEvent $event)
    {
        $locale = null;
        $request = $this->container->get('request');
        $session = $event->getRequest()->getSession();
        
        if (null !== $session)
        {
            $locale = $session->getLocale();
        }
        
        if (
            $request->query->has('lang') &&
            in_array($request->query->get('lang'), $this->container->getParameter('locales'))
        )
        {
            $locale = $request->query->get('lang');
        }
            
        if (
            $request->request->has('lang') &&
            in_array($request->request->get('lang'), $this->container->getParameter('locales'))
        )
        {
            $locale = $request->request->get('lang');
        }
        
        if($locale !== NULL)
        {
            $this->listener->setTranslatableLocale($locale);
        }
    }
}

Router.php

<?php
 
namespace ACME\ExampleBundle\Routing;

use Symfony\Component\Routing\Router as BaseRouter;
use Symfony\Component\Routing\RequestContext;
use Symfony\Component\DependencyInjection\ContainerInterface;

class Router extends BaseRouter
{
        protected $container;

    /**
     * Constructor.
     *
     * @param ContainerInterface $container A ContainerInterface instance
     * @param mixed              $resource  The main resource to load
     * @param array              $options   An array of options
     * @param RequestContext     $context   The context
     * @param array              $defaults  The default values
     */
    public function __construct(ContainerInterface $container, $resource, array $options = array(), RequestContext $context = null, array $defaults = array())
    {
        $this->container = $container;

        $this->resource = $resource;
        $this->context = null === $context ? new RequestContext() : $context;
        $this->defaults = $defaults;
        $this->setOptions($options);
    }

    /**
     * {@inheritdoc}
     */
    public function getRouteCollection()
    {
        if (null === $this->collection) {
            $this->collection = $this->container->get('routing.loader')->load($this->resource, $this->options['resource_type']);
        }

        return $this->collection;
    }
    
    /**
     * Generates a URL from the given parameters.
     *
     * @param  string  $name       The name of the route
     * @param  mixed   $parameters An array of parameters
     * @param  Boolean $absolute   Whether to generate an absolute URL
     *
     * @return string The generated URL
     */
    public function generate($name, $parameters = array(), $absolute = false)
    {
        $request = $this->container->get('request');
        $locales = $this->container->getParameter('locales');
        
        if(
                $request->request->has("lang") &&
                in_array($request->request->get("lang"), $locales) &&
                (! isset($parameters['lang']) || ! in_array($parameters['lang'], $locales))
        )
        {
                $parameters['lang'] = $request->request->get("lang");
        }
        if(
                $request->query->has("lang") &&
                in_array($request->query->get("lang"), $locales) &&
                (! isset($parameters['lang']) || ! in_array($parameters['lang'], $locales))
        )
        {
                $parameters['lang'] = $request->query->get("lang");
        }

        return $this->getGenerator()->generate($name, $parameters, $absolute);
    }
}

services.yml

#should be in your bundle (ACME\ExampleBundle\Resources\config\services.yml)
parameters:
    router.class:    ACME\ExampleBundle\Routing\Router

services:
    ACME.listener.translatable:
      class: ACME\DosingGuideBundle\Listener\TranslationListener
      arguments: [ @gedmo.listener.translatable, @service_container ]
      tags:
       - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest }