Usage:

$app = new Application();
$app->register(new TranslationExtension(), array(
    'translation.class_path' => '...',
));

$app['translator.messages'] = array(
    'en' => array(
        'apple' => 'apple',
    ),
    'fr' => array(
        'apple' => 'pomme',
    ),
);

$app['locale'] = 'fr';

echo $app['translator']->trans('apple');

If you use the Symfony Bridge extension, some tags and filters are also
automatically available in templates:

$templates = array(
    'index' => '{% trans %}apple{% endtrans %}'
);