I am from Brazil, and although I understand the "dangers" of this idea (like community fragmentation, and why not other widely spoken languages, like Hindi) I really like the initiative.
Although as you can see from this text, I can read and write english, I am a absolute minority in Brazil, most people here don't understand english at ALL, just look at the map in the post, they had 3.4M Brazillian visitors, and 1M Portuguese visitors, but Brazil has 200 million people, portugal has 10 million people, this means that Brazil with literally 20 times more people than Portugal, can only have 3 times the visits on a english speaking site.
Compare this to Brazil own internet use, Brazil is one of the countries with widest internet use, its internet culture resemble Japan and China, where it has a very strong "local" internet, because people don't know any other foreign language.
Some people might say: "But to code you must know english!" Well, you don't, I started coding when I was 6 years old, taught by my dad, that still does not know english, what happen in those cases, is that keywords (like goto, print, etc...) are just that, keywords, they are like mantra, enchantments, magic words that you type, and the computer do something, when I learned GWBASIC I had no idea what "if, else, then" meant, except they were mandatory when I wanted to control program flow, I had no idea what "GOTO" means, except that it made the program jump to somewhere else, the first time I saw "GOTO" written as "go to" I was amazed to learn that it was actually two words!
And this still stands, my dad, and most of my co-workers (I worked once in a company with 40 programmers, only 4 knew english), they might even search stuff in google in english, but usually they do this for example by copying and pasting errors from the compiler, the error is in english, but they have no idea what is the literal meaning or translation, and after the search is done, they use google translate.
Portuguese language programming groups are very strong here, for example I am in the C++, Android, Lua and some other mailing lists, there is almost no connection between those lists and the english language ones, because the overlap is too thin, few people in the portuguese speaking lists know english, and the ones that DO know english, keep themselves to english lists because of higher volume and content.
I expect that pt.so will take some time to reach the original SO quality, but I also expect it to mean much for brazillian community.
And curse you SO devs, I wanted to make that as my startup! :P (I am half-serious, if I had time I would have made PT SO myself)