Checkout option translation + proofreading

As of the OHT api, we can as for a translation + proofreading project.

    /**
     * Create a new Translation + Proofreading
     * @param string $source
     * @param string $target
     * @param string $sources
     * @param integer $word_count (optional)
     * @param string $notes (optional)
     * @param string $callback_url (optional)
     * @param array $params (optional)
     * @return stdClass response object
     */