Yep, I built it. It depends. If you choose one of the default LibreTranslate servers, then you do not need an API key. They are free services, their uptime may not be the best so try rotating through them.
If you use DeepL, you will need to sign up for an API account to get an API key to paste into Damus settings. The free plan has been sufficient for my usage.
Prompts to translate notes will show up if you’ve tapped into the note.