public function sync()
    {
        $this->backup();

        $output = [];

        $translations = $this->getTranslations();

        $keysFromFiles = array_collapse($this->getTranslationsFromFiles());

        foreach (array_unique($keysFromFiles) as $fileName => $key) {
            foreach ($translations as $lang => $keys) {
                if (! array_key_exists($key, $keys)) {
                    $output[] = $key;
                }
            }
        }

        return array_values(array_unique($output));
    }