use Symfony\Component\Translation\Translator;
use Symfony\Component\Translation\MessageSelector;
use Symfony\Component\Translation\Loader\ArrayLoader;

$translator = new Translator('fr_FR', new MessageSelector());
$translator->setFallbackLocales(array('fr'));
$translator->addLoader('array', new ArrayLoader());
$translator->addResource('array', array(
    'Hello World!' => 'Bonjour',
), 'fr');

echo $translator->trans('Hello World!')."\n";