Drupal has pretty good multilingual support out of the box. It's also fairly easy to create new entities and just add translation support through the annotation. These things are well documented elsewhere and a quick search will reveal how to do that. That is not what this post is about. This post is about the UX around selecting which fields are translatable.
Part of me is suspecting that I may be one of the lucky 10,000 today but I figure it's worth putting this out there because if I wasn't aware of this then there may be others too. It turns out that the version of Drush that you just installed may not be the version of Drush that executes your command.