Ensure language is valid

- Do not override locale if user has choosen a locale from the login screen.
- Add some tests about locale url

src/Wallabag/CoreBundle/Event/Listener/UserLocaleListener.php

@@ -6,8 +6,10 @@ use Symfony\Component\HttpFoundation\Session\Session;
 use Symfony\Component\Security\Http\Event\InteractiveLoginEvent;
 
 /**
- * Stores the locale of the user in the session after the
- * login. This can be used by the LocaleListener afterwards.
+ * Stores the locale of the user in the session after the login.
+ * If no locale are defined (if user doesn't change it from the login screen), override it with the user's config one.
+ *
+ * This can be used by the LocaleListener afterwards.
  *
  * @see http://symfony.com/doc/master/cookbook/session/locale_sticky_session.html
  */
@@ -30,7 +32,7 @@ class UserLocaleListener
     {
         $user = $event->getAuthenticationToken()->getUser();
 
-        if (null !== $user->getConfig()->getLanguage()) {
+        if (null !== $user->getConfig()->getLanguage() && null === $this->session->get('_locale')) {
             $this->session->set('_locale', $user->getConfig()->getLanguage());
         }
     }