According to Chinese tech site DoNews (sketchy English translation here), Apple plans on bringing support for Mandarin Chinese to Siri as early as March of this year. Other dialects of Chinese, such as Cantonese, will remain unsupported for the time being. Support for Japanese and Russian is supposedly coming at the same time.
Although the source may sound somewhat suspicious -- a Chinese tech site that claims to have inside information from an Apple engineer working on Siri -- this information does align with Apple's own promise to increase Siri's linguistic acumen in 2012. Apple has also promised to bring support for Spanish, Italian, and Korean to Siri sometime this year.
Siri will supposedly also offer local business information and navigation options to Chinese users, which is something that's been US-only so far. With China being Apple's biggest emerging market, it's obvious why Apple might want to prioritize increased Siri functionality in that country. (As for me and the rest of the New Zealand iOS users, we can probably look forward to full business/navigation support in Siri sometime in 2022.)
These updates to Siri's linguistic capabilities are likely to launch alongside an iOS update, probably iOS 5.1. Like many others, Stephen Millward of Penn-Olson speculates the iOS 5.1 update will launch alongside new iPad hardware; there's obviously no way to know that for certain, but my gut tells me he's probably right.