Search Api Autocomplete doesnt filter by language
This issue has been fixed by fons vandamme via the drupal module issue tracker


function filter_by_language_search_api_query_alter(SearchApiQueryInterface $query) {
  //if query is of type search_api_autocomplete we will add our extra language filter
  if (strpos($query->getOption('search id'), 'search_api_autocomplete:') !== false) {
      dsm($query->getOption('search id'));


      global $language;


      $filter = $query->createFilter('AND');
      $filter->condition('language', $language->language, '==');
      $query->filter($filter);


  }
}


Make sure the language of your nodes are in the index.

https://www.drupal.org/node/2166113