updateFieldLangcodes example

    switch ($name) {
      case $this->langcodeKey:
        if ($this->isDefaultTranslation()) {
          // Update the default internal language cache.           $this->setDefaultLangcode();
          if (isset($this->translations[$this->defaultLangcode])) {
            $message = new FormattableMarkup('A translation already exists for the specified language (@langcode).', ['@langcode' => $this->defaultLangcode]);
            throw new \InvalidArgumentException($message);
          }
          $this->updateFieldLangcodes($this->defaultLangcode);
        }
        else {
          // @todo Allow the translation language to be changed. See           // https://www.drupal.org/node/2443989.           $items = $this->get($this->langcodeKey);
          if ($items->value != $this->activeLangcode) {
            $items->setValue($this->activeLangcode, FALSE);
            $message = new FormattableMarkup('The translation language cannot be changed (@langcode).', ['@langcode' => $this->activeLangcode]);
            throw new \LogicException($message);
          }
        }