What does "written in various languages" mean? Mixing multiple legacy encodings in one filename? Unicode filenames work fine on Linux (I just tested it with Chinese) and I would be very surprised if they didn't on Windows.
I don't know if that's still an issue.
As an aside, it'd be perfectly legitimate for Mercurial (and git) to reject filenames that aren't representable in unicode.
I was surprised because we've been using it for years at work, sharing repositories between Linux and Windows, and we have never been bitten by this. Maybe the latin-1 subset of Unicode works? Or all the files we worked on just happened to have ASCII names (despite us being mostly non-Americans).