Search on multi-language sites to search all languages

How can I search all languages in a multi language site? For instance while browsing in the german version of the website the search should also display results from the english version. How can I do this?

So far Iā€™m using the controller code from the kirby docs.

<?php 

return function($site, $pages, $page) {

  $query   = get('q');
  $results = $site->index()
          ->visible()
          ->search($query, 'title|work_date|place|city|country');

  return array(
    'query'   => $query,
    'results' => $results, 
  );

};

Thank you.