Yup, totally centralized. Spent half an hour figuring out where my question should go (StackOverflow vs Programming vs ServerFault vs SuperUser vs Ubuntu vs Unix-and-Linux). Ended up not asking the question at all.
Shall they open some foreign embassies, I guess users'll spend 10 minutes more deciding in which language should they ask (i.e. whenever they prefer native language or bigger community) or duplicate the question.
And it's really not that hard in most cases. Is it an objective question about writing code? It goes on Stack Overflow. Is it about programming as a broader topic? It goes on Programmers. Is it about servers or systems administration? It goes on ServerFault. Is it about general computing? It goes on SuperUser. Is it a highly subjective question about programming? Stack Exchange is not interested in your question.
(I do agree that SO and Programmers seem more like an artificial division than natural categories, but I still don't find it that hard to figure out whether a question is good for SO.)
That would depend on why help was needed--is it a problem with expressing a certain programming concept in Bash? or with portability across various Unixen? Or with the proper way to invoke some server managament program?
If someone genuinely doesn't know which stackexchange community has the most relevant expertise to help with a problem, there's a mechanism in place to handle that; just post a question somewhere and see if it gets migrated or closed as "belongs on xxx".
There is also some overlap, so if you have a question about Ubuntu you can also ask it on Unix&Linux.
The important point is that the moderation should encourage the participation of the not native speakers. If someone post a good question or answer with a few grammatical mistakes, then the expected behavior should be to fix the text and not mock the author.
If you feel like wanting to give back to the community, answer in your native language site, if it exists.
[Disclaimer: I’m a native Spanish speaker, ES-AR to be more precise.]