Can't play Diablo 3 in different language. Why?
Blizzard announced that the retail versions of Diablo III would be locked to the region language, so if you bought yours in Brazil, your key is a Latin key and you'll only be able to play it in Brazilian Portuguese or Spanish.
Blizzard said that they made it this way in order to prevent what you did, to buy a cheaper game in another region (Brazil, Russia for example) and play the game in your natural language.
You can only choose your language freely in the special edition retail and the digital versions.
Here's an article that explains this further.