We already have the Cyrillic letters done, actually: https://github.com/The-Powder-Toy/The-Powder-Toy/tree/font-cyrillic

It's not in the main game yet because there's other stuff to do first, like updating the TPT server to not block unicode. If we added them to the game, it would reject all save names and comments containing these letters.

There's three parts to this story.

First is the support of unicode text throughout the game. This is an enormous step but we've already done this quite a while ago actually! (snapshot 132, v94.0) But currently there is no way to enter unicode text into the game, and also the server will reject any save titles, comments, etc containing unicode characters.
Developer sidenote: you can verify that it's there if you type

string.char(0xEE,0x80,0x92)

in the console -- this is the UTF-8 byte sequence for U+E012, a private use unicode character that we use for the "new save" icon.

The second part is the font characters themselves. Myself and @savask have already drawn them, and I've just finished drawing the rest of the

Cyrillic: U+0400–U+04FF

block, and merged that branch; but they aren't in any release yet. @jacob1 gave an OK for merging any complete font blocks, so we also got Latin-1 now. Next release/snapshot you should be able to type both into the game. But the server will still reject any saves or comments. That part is still undecided.

I'm assuming you're interested in contributing translations for the game so there's a third part to the story: the actual translation data for all of the text in the game. We currently have no infrastructure in the code to handle translations, we would need to sit down and figure out how to proceed with this, or even if we want to at all? Until then and until we decide on a format for translation data I don't think there's any useful way in which you can help translate the game. (Other than, like, making a fork of the game and replacing all the text, but maintaining such a fork will be quite a pain).

P.S. By the way here are the glyphs we came up with

P.P.S. Latin-1 block