Well, unless you do join spanish speaking lobbies in EU servers, i do not get why they get riled up.
English is not my first language, it is spanish, i do live and play wow from Spain in US servers, and i can tell you that in US servers, the spanish talking population it is far more used to english talking people than for example a Spanish speaking lobby from Spain playing in EU servers.
No one i have played with in US spanish servers would be riled because of someone talking in english.
Another subject is how riled are English talking players when they listen to me talking in english in Voice IP programs, somehow my accent is like Alien to them, and they can not help but asking me where i am from because they do not have a clue.