Pimcore\Model\Document\Service\Dao::addTranslation PHP Method

    public function addTranslation(Document $document, Document $translation, $language = null)
    {
        $sourceId = $this->getTranslationSourceId($document);
        if (!$language) {
            $language = $translation->getProperty("language");
        }
        $this->db->insertOrUpdate("documents_translations", ["id" => $translation->getId(), "sourceId" => $sourceId, "language" => $language]);
    }