EntityDecorator.php

<?php

  public function __construct(array $values, $entity_type) {
    $class = self::getDecoratedClass($entity_type)
    $this->subject = new $class($values, $entity_type);
  }
  protected static function getDecoratedClass(string $entity_type = NULL): string {
    return \Drupal::entityTypeManager()
      ->getDefinition($entity_type ?? self::ENTITY_TYPE)
      ->get(__CLASS__);
  }

  protected static function translationHelper(ContentEntityInterface $original_translation) {
    $translation = new self([], $original_translation->getEntityTypeId());
    foreach ($original_translation->toArray() as $field_name => $lists) {
      $item_list = $translation->$field_name;
      foreach ($lists as $delta => $values) {
        $item_list->set($delta, $values);
      }
    }
    return $translation;
  }

  /**
   * {@inheritdoc}
   */
  public function getTranslation($langcode) {
    return self::translationHelper($this->subject->getTranslation($langcode));
  }

  /**
   * {@inheritdoc}
   */
  public function getUntranslated() {
    return self::translationHelper($this->subject->getUntranslated());
  }

  /**
   * {@inheritdoc}
   */
  public function addTranslation($langcode, array $values = []) {
    return self::translationHelper($this->subject->addTranslation($langcode, $values));
  }

  /**
   * {@inheritdoc}
   */
  public static function load($id) {
    $class = static::getDecoratedClass();
    return $class::load($id);
  }