A really bad idea. Now a really good answer or tip that would otherwise be on the 'regular' stackoverflow (albeit in broken English) will remain, unknown, on a 'pt.overflow.' That is one drawback. Another drawback, an English only stackoverflow helps a Portuguese speaking person to learn another language that is, now, more universal. Yet another, it fractures an otherwise more unified community. My mother tongue is S…
This assumes that, had the Portuguese site not existed, the question would have been answered on the English site. There is also the possibility that the question would not have been answered at all.