Are you talking about the native Chinese MIUI? They've said that they only support Chinese, because they make their rom specifically for a Chinese audience.
If you're asking about why miui-dev.com doesn't support other languages, then you must not have looked HERE.
I think you'd better ask people how you could translate the whole thing into your language. Mark should be very knowledgeable on this as he's the founder of miui-dev and did the chinese-> english translation every week.
Sent from my Nexus One using the miui-dev.com Forums App