https://en.wikipedia.org/wiki/List_of_programming_languages

and thought it would be interesting to do some work in some of them over time, since some of them have some unusual / powerful features. (Side projects can obviously be done, and I am doing one or two, but that is not my focus in this question.)

I know I can google for "$LANGUAGE_NAME jobs", but am interested to hear if anyone knows such details about any of those languages outside the mainstream. Mainstream ones, I guess, would include COBOL (mainframes), Pascal (Delphi), Lisp(s) and Schemes, C, C++, Java, Python, Perl, PHP, Ruby, C#, VB.NET and some others too, though feel to correct / add / etc. Sorry if I missed out anyone's favorite language or one considered mainstream - I don't know them all. (Not sure what Haskell / Clojure / Scala / Erlang / Elixir / Golang count as, though I've read about usage of all of them in recent years, and some for longer.) Anyway, "mainstream" is subjective (but this is not StackOverflow, where they close such subjective questions as off-topic, so I'm hoping I'll get some answers).

On a (not so) side note, if anyone here has done work in any of those non-mainstream languages, it would be interesting to hear about the projects too, if not confidential.

I've tried out at least a handful of non-mainstream languages in the past, but have not done any non-trivial project in them, that I can remember. Some of the ones I've tried are Forth, Elastic, Pike, and Icon.