Its just a matter of common courtesy for crying out loud. I speak spanish, but when spanish speaking coworkers start conversations in spanish, I immediately shift it into english.
Thank you.
I tend to respond to Spanish being thrown at me ... with Chinese! That usually leads to a conversation about being courteous and not excluding people.
But then again, I speak four languages so I have a lot of opportunity to create havoc among people who "just don't get it".