American sign language or English in my head.
I have had many completely deaf friends and took 2 years of American sign language in high school for the fear of actually losing my own hearing due to the amount of ear infections I kept getting.
I can only guess but from my completely deaf friends they told me there thought were those of us hearing folks. So I guess It would depend on what country you lived in as to what language you would think in.
My question to you is how you would this when you can hear?