Views default options are not translated when creating a foreign language view

Problem/Motivation

1. Install Drupal in German.
2. Create a view. Look at pager settings, they are in English. Save it.
3. All strings are in English but the file is langcode: de.

We don't t() the default options. For example:

abstract class ExposedFormPluginBase extends PluginBase {

  protected function defineOptions() {
    $options = parent::defineOptions();
    $options['submit_button'] = array('default' => 'Apply');
    $options['reset_button'] = array('default' => FALSE);
    $options['reset_button_label'] = array('default' => 'Reset');
    $options['exposed_sorts_label'] = array('default' => 'Sort by');
    $options['expose_sort_order'] = array('default' => TRUE);
    $options['sort_asc_label'] = array('default' => 'Asc');
    $options['sort_desc_label'] = array('default' => 'Desc');
    return $options;
  }
}

abstract class SqlBase extends PagerPluginBase {

  protected function defineOptions() {
    $options = parent::defineOptions();
    $options['items_per_page'] = array('default' => 10);
    $options['offset'] = array('default' => 0);
    $options['id'] = array('default' => 0);
    $options['total_pages'] = array('default' => '');
    $options['expose'] = array(
      'contains' => array(
        'items_per_page' => array('default' => FALSE),
        'items_per_page_label' => array('default' => 'Items per page'),
        'items_per_page_options' => array('default' => '5, 10, 25, 50'),
        'items_per_page_options_all' => array('default' => FALSE),
        'items_per_page_options_all_label' => array('default' => '- All -'),

        'offset' => array('default' => FALSE),
        'offset_label' => array('default' => 'Offset'),
      ),
    );
    $options['tags'] = array(
      'contains' => array(
        'previous' => array('default' => '‹ previous'),
        'next' => array('default' => 'next ›'),
      ),
    );
    return $options;
  }
}

The options are documented to have a 'translatable' key but its not clear how that should be used or what it means. It does not seem to be used either.

  /**
   * Fills up the options of the plugin with defaults.
   *
   * @param array $storage
   *   An array which stores the actual option values of the plugin.
   * @param array $options
   *   An array which describes the options of a plugin. Each element is an
   *   associative array containing:
   *   - default: The default value of one option
   *   - (optional) contains: An array which describes the available options
   *     under the key. If contains is set, the default will be ignored and
   *     assumed to be an empty array.
   *   - (optional) 'translatable': TRUE if it should be translated, else FALSE.
   *   - (optional) 'bool': TRUE if the value is boolean, else FALSE.
   */
  protected function setOptionDefaults(array &$storage, array $options) {

Proposed resolution

Translate default options in their definitions.

Remaining tasks

Discuss. Review.

User interface changes

None.

API changes

The defaultOptions() expected return value slightly changes.