Translate the Flash Message
Presuming you are in a Controller:
$request->getSession()->getFlashBag()->add( 'notice', $this->get('translator')->trans('contact.message.email_has_been_sent'));Read how to handle Translations.
Alternatively, in twig:
{% for flashMessage in app.session.flashbag.get('notice') %} <p>{{ flashMessage|trans }}</p>{% endfor %}