I fixed this today. You should get an error when you try to save an email template with missing interpolation keys.
https://github.com/discourse/discourse/commit/1a378126259b1461bb935d1385778842d33b088a
Bonus: Changing an email template will create an entry in the Staff Log. That didn’t happen before.
While testing this I also noticed that reverting changes to an email template or site text can result in a “translation missing” error. I think I found a solution for that as well, but I’m not comfortable merging it in until someone else from the team takes a look at it.
@Pad_Pors Unfortunately it’s currently not possible to omit interpolation keys.