If you should run into the same problem as I did and wonder that your AJAX callback results return in the wrong language, you may have missed to add the language (path) prefix to the AJAX call!
AJAX calls determine the language from the path they were called, if language prefixing method is used in Drupal.
'/' + Drupal.settings.pathPrefix + 'myCallbackProviderName/' + ...;
Then the language will be determined just fine in your Drupal AJAX calls.
So don't forget to language-prefix your AJAX calls!! ;)