class LanguageItem

class LanguageItem extends FieldItemBase {

  /**
   * Array of property definitions of contained properties.
   *
   * @see PropertyEntityReferenceItem::getPropertyDefinitions()
   *
   * @var array
   */
  static $propertyDefinitions;

  /**
   * Implements ComplexDataInterface::getPropertyDefinitions().
   */
  public function getPropertyDefinitions() {
    if (!isset(self::$propertyDefinitions)) {
      self::$propertyDefinitions['value'] = array(
        'type' => 'string',
        'label' => t('Language code'),
      );
      self::$propertyDefinitions['language'] = array(
        'type' => 'language',
        'label' => t('Language object'),
        // The language object is retrieved via the language code.
        'computed' => TRUE,
        'read-only' => FALSE,
        'settings' => array(
          'langcode source' => 'value',
        ),
      );
    }
    return self::$propertyDefinitions;
  }

  /**
   * Overrides FieldItemBase::setValue().
   */
  public function setValue($values) {

    // Treat the values as property value of the object property, if no array
    // is given.
    if (!is_array($values)) {
      $values = array(
        'language' => $values,
      );
    }

    // Language is computed out of the langcode, so we only need to update the
    // langcode. Only set the language property if no langcode is given.
    if (!empty($values['value'])) {
      $this->properties['value']
        ->setValue($values['value']);
    }
    else {
      $this->properties['language']
        ->setValue(isset($values['language']) ? $values['language'] : NULL);
    }
    unset($values['language'], $values['value']);
    if ($values) {
      throw new InvalidArgumentException('Property ' . key($values) . ' is unknown.');
    }
  }

}