There's a surprisingly big list of languages not supported, just off the top of my head: Standard ML, OCaml, Rebol, Elm, PureScript, Racket, Lisp-flavored Erlang, Common Lisp, Zig, Perl 5.
Most of these even have JavaScript targets already.
Edit: OCaml is supported through Reason Node.js on BuckleScript.
I agree they should be there. We've been more focused more on vertical feature work than horizontal. But we're hiring a language hacker if anyone wants to help us get more language and tooling support https://repl.it/@util/jerbs#open-positions/language_hacker.t...
Most of these even have JavaScript targets already.
Edit: OCaml is supported through Reason Node.js on BuckleScript.