I'll second that unpopular opinion and claim I have experience with many other languages that I also enjoy.

The fun part about Mumps is that it's basically Lisp's ugly sibling, without meaningful scoping rules. You can easily do lots of terrible things in Mumps, but those things are usually more straightforward than the terrible things you do in other languages. You can also do elegant things, though a little less straightforwardly.

My single biggest complaint about Mumps codebases is that they're usually old, which means they have shitty documentation. Not that new ones are necessarily any better, but at least there's a chance new things are written in Orgmode or something :)