Code injection in the way Symfony implements translation caching in FrameworkBundle

When investigating issue [#11093](https://github.com/symfony/symfony/issues/11093), [Jeremy Derussé](https://connect.sensiolabs.com/profile/jderusse) found a serious code injection issue in the way Symfony implements translation caching in FrameworkBundle.

- Your Symfony application is vulnerable if you meet the following conditions:

- You are using the Symfony translation system from FrameworkBundle (so basically if you are using Symfony full-stack -- you are not affected if you are using the Translation component with Silex for instance);
You don't sanitize locales coming from a URL (any route with a _locale argument for instance):

When vulnerable, an attacker can submit a non-valid locale value that can contain some PHP code that will be executed by Symfony. That's because the locale value is dumped into a PHP file generated in the cache without being sanitized first.