Add translation files to custom module doesn't work
You might need to uninstall the module completely and reinstall it again, in order to be able to import the translations. Otherwise, they are not picked up automatically. There are a couple of threads in the Drupal forums mentioning this issue: https://www.drupal.org/node/220341 , https://www.drupal.org/node/435706#comment-2711736 

Hope that helps,