I sent 2 invitations to my classmates and unfortunately the email is in Chinese. Maybe it's because my bitbucket account's locale is set to Chinese(China).
I didn't see any option to choose the language of the invitation email so I thought it should be in English. And of course I received a relply from one of them telling me that the invitation email is not understandable. So I changed my account settings and sent invitations again. I am not sure whether it is in English this time because I haven't received any feedback.
Now I changed my account setting back to Chinese again. By choosing Chinese in my account setting, I am not meaning that all my friends speak Chinese. So I think this place might be changed to make the site more usable.