What languages or engines are allowed? Anything is allowed
That said, you'll have to "post" the game to itch.io to submit it -- I see no reason that can't be a web game, a MacOS downloadable game, Windows game, etc. The caveat being that Web games are generally more accessible and you're likely to get the most plays with something easy for people to try.
Thanks for covering this, bitDecay! I can confirm that's correct. You can use any game engine and publish in any format - but be aware that people can only rate games that they can play. I wouldn't recommend submitting your game as a ZX-81 cassette tape format, for example.
