When you go into the Account section and the Courses tab, at the top right should be a "Manage" button. This allows you to not delete, but archive languages so they don't appear anymore but your progress is saved. This is for web though, I don't know about the mobile app.
Posts made by Asraelite
RE: Delete a course I added by mistake
RE: Does Lingvist continually test you when presenting new words?
I don't think it'll appear again. As a rule I avoid guessing words for this reason, even if I'm fairly sure I would get the guess correct.