Language is more than just letter written differently. They also convey a culture, shared knowledge and very often they also connect people shared problems.

For example what is happening in Brazil is quite a problem, people outside that region would be likely to care less or nothing, whereas the people around would try to help each other and +95% brazilians just speak portuguese, so they'd be bound to only want topics tagged with PT language.

The tag doesn't even need to be added manually. Nowadays is fairly trivial to automatically recognize the language and add the tag. 

 I think a better option would be to be able to chose to separate posts for region, and an option in the clients that would automatically translate all foreign language posts to a chosen language. 

 Approximate geographical locations. Assuming it would be voluntary and optional, would definitively be a good way to grow local groups.