Can’t We All be Reasonable and Speak English? is the tongue-in-cheek title of the announcement of
Stack Overflow em Português, "é um site de perguntas e respostas para programadores profissionais e entusiastas". It launched in February and is doing well as far as I can tell, despite early worries about fragmentation of knowledge base. From the announcement:
So know this: we are at least as worried about fragmentation as you are. And we have a plan:
Eventually, all of you are going to have to learn Portuguese.
An obvious prerequisite for the existence of an SE site in a language other than English: Stack Exchange must have a Community Manager fluent in the language. For the Portuguese site they hired Gabe, The Smiling Community Manager. They recently added jmac, Community Manager of the Rising Sun and are still looking for two bilingual CMs: English/Japanese and English/Spanish.
As Jonas Meyer said, there are proposals for Math in Korean and in Spanish. Given that SE wants to add a Spanish-speaking CM already, it seems that the Spanish language proposal has a better chance of becoming reality soon. So far, its level of support on Area51 remains low (but it might increase if Stack Overflow in Spanish is created).
It's easy to identify areas that need a SE site, quite another to actually gather the right people into creating a site. -- Shog9