Output ctools_modal_form_wrapper() with translatable items in the form
This is an open issue for ctools modal_form and is in need of some more data / feedback to push the issue forward. The specific bug is called out here and appears to be related to the language header that is used in the modal form window:

https://www.drupal.org/node/2160293

There is a potential patch for this here:

https://www.drupal.org/node/2245123