prepareTranslation example

    if (!$entity->isDefaultRevision() && ContentTranslationManager::isPendingRevisionSupportEnabled($entity_type_id, $entity->bundle())) {
      /** @var \Drupal\Core\Entity\ContentEntityStorageInterface $storage */
      $storage = $this->entityTypeManager()->getStorage($entity->getEntityTypeId());
      $revision_id = $storage->getLatestTranslationAffectedRevisionId($entity->id(), $source->getId());
      if ($revision_id != $entity->getRevisionId()) {
        $entity = $storage->loadRevision($revision_id);
      }
    }

    // @todo Exploit the upcoming hook_entity_prepare() when available.     // See https://www.drupal.org/node/1810394.     $this->prepareTranslation($entity, $source, $target);

    // @todo Provide a way to figure out the default form operation. Maybe like     // $operation = isset($info['default_operation']) ? $info['default_operation'] : 'default';     // See https://www.drupal.org/node/2006348.
    // Use the add form handler, if available, otherwise default.     $operation = $entity->getEntityType()->hasHandlerClass('form', 'add') ? 'add' : 'default';

    $form_state_additions = [];
    $form_state_additions['langcode'] = $target->getId();
    $form_state_additions['content_translation']['source'] = $source;