I had an interview for the Fulbright to Mexico about six weeks ago. It was in Spanish (which I'm not fluent in, but conversational). Pretty intimidating, but basically I just set up a google doc and wrote down every question I thought I was likely to be asked about my proposal and my reasons for pursuing the grant. After writing answers to those questions (which actually helped me clarify my project in and of itself) I translated everything into Spanish and just practiced, practiced, etc. Also found people to practice with, and had a kind of "mock interview" set up through (Skype and all) with a few of my professors.
I can't say whether your interview would be in English or not, it would help to ask because that makes a huge difference. I hope that helps. Good luck!