i18n is a technical issue. What matters most in this case is not that StackOverflow does not have a Mandarin interface, but rather, it does not have a Mandarin community.
Well, which comes first, chicken or the egg? The language support or the community?
Without the community there's no need to support the language. But without the language you cannot support the community.
Part of the problem is that localization can be expensive (especially for non-Western languages) and there is no easy way for community driven sites to gauge potential returns of investing in a new language. But the fact that someone took the trouble of cloning the whole site probably indicates that there's interest out there.