How to translate strings in (programmatically created) custom module?
Potx is not yet ported to Drupal 8 but it can parse Drupal 8 modules.

You can install it as a drush command instead of a drupal module and then it should work fine on your Drupal 8 module.

What that doesn't support yet is extracing existing translations from a running site. Looks like people are working on a port in https://www.drupal.org/node/2356469 which supports that.