Hello jane.gannaway, and welcome to the forum!
The tour is lead by a live guide speaking so the narration will be in the language listed for the tour.
Funnily enough I took the tour in French 20+ years ago because they did not have an English option at the time, but I really only speak tourist French and missed a lot of what they were saying. Early on the guide did try and throw in some English information for us since he knew our French was bad but in the more complicated discussions of affairs of state I did not follow them.
I took the English tour a couple of years ago and I don't know if I would recommend doing it in a language you don't speak. Years ago you got to see many rooms unavailable without the special tour but now much more of the Palace is generally open to the public. I think the information helps tremendously with context about Venice, politics and the Doge, but I might recommend spending the money on a guided tour of the Palace itself versus taking the secret tour in a language you don't understand.
Hope that helps,
=Tod