public function assertTranslation($content_type, $title, PyStringNode $string) {
    $values = $this->getYamlParser()->parse($string);
    Assert::keyExists($values, 'langcode', __METHOD__ . ": Required field 'langcode' not found.");

    $nid = $this->getCore()->getEntityIdByLabel('node', $content_type, $title);
    $source = $this->getCore()->entityLoad('node', $nid);

    $this->getCore()->entityAddTranslation($source, $values['langcode'], $values);
  }