// src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

class FrameworkExtension extends Extension {
...
 private function registerTranslatorConfiguration(array $config, ContainerBuilder $container, LoaderInterface $loader) {
...
    if (\array_key_exists('cache_dir', $config)) {
            $defaultOptions = $translator->getArgument(4);
            $defaultOptions += ['cache_dir' => $config['cache_dir']];
            $translator->setArgument(4, $defaultOptions);
        }
    }
...
}