Allow default_langcode field value to be changed

In case anyone might find this useful, this code will remove the edit-langcode element from ContentEntityForms if there are already translations created to prevent the error "Can't change the default langauge"

/**
 * Implements hook_form_alter().
 */
function yourmodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $form_object = $form_state->getFormObject();
  if ($form_object instanceof ContentEntityFormInterface) {
    // Prevent switching languages on already translated content.
    // @see \Drupal\content_translation\ContentTranslationHandler::entityFormAlter.
    $entity = $form_object->getEntity();
    if ($entity->isTranslatable() && count($entity->getTranslationLanguages()) > 1) {
      // Translations exist, prevent language switching.
      $langcode_key = $entity->getEntityType()->getKey('langcode');
      $form[$langcode_key]['widget']['#access'] = FALSE;
    }
  }
}

But additionally we need constraints that ensure that there is always a valid default language.

— @mkalkbrenner in #3, >8 years ago.

This is now within reach: ✨ Add a `langcode` data type to config schema Fixed is RTBC, and then we'll still need to do the work here to actually run validation constraints 😊

@Wim: This is about content entities and the boolean default_langcode flag. (ensuring that exactly one translation has this flag). That has nothing to do with langcode validation in config entities.

Mea culpa — I thought it was about a default_langcode setting in some Field(Storage)Config. 🙈

That being said … exactly the same validation approach will be usable here AFAICT? 😊 That'd be:

constraints:
  NotNull: []
  Choice:
    callback: 'Drupal\Core\TypedData\Plugin\DataType\LanguageReference::getAllValidLangcodes'

That's also why I didn't dig very deep into this issue, because that alone landing in core in another issue would be sufficient to make this issue more actionable?

✨ Add a `langcode` data type to config schema Fixed landed. Per #43, AFAICT that should still help this issue! 😊