I noticed that in the new version of the game I can type in Cyrillic and it's displayed correctly, but if I post a save, the letters are replaced by a "?". To the point of the question, can new languages be added to the game?
Not quite, not yet. As per this comment, the in-game font is on the way to supporting most major writing systems, but it'll take a while for these to be usable on the website.