LanguageAddForm::validateCustom

public LanguageAddForm::validateCustom(array $form, FormStateInterface $form_state)

public function validateCustom(array $form, FormStateInterface $form_state) {
  if ($form_state->getValue('predefined_langcode') == 'custom') {
    $langcode = $form_state->getValue('langcode');
    // Reuse the editing form validation routine if we add a custom language.
    $this->validateCommon($form['custom_language'], $form_state);

    if ($language = $this->languageManager->getLanguage($langcode)) {
      $form_state->setErrorByName('langcode', $this->t('The language %language (%langcode) already exists.', array('%language' => $language->getName(), '%langcode' => $langcode)));
    }
  }
  else {
    $form_state->setErrorByName('predefined_langcode', $this->t('Use the <em>Add language</em> button to save a predefined language.'));
  }
}