I lived there for two years, and I think I met one french speaking expat during that time. Czech is the official language (surprise!), and “everyone” speaks it (apart from yours truly).
One thing I found interesting while living there was that it was usually a safe bet to assume everyone older than me knew russian as a second language, while everyone my age or younger spoke english.
I lived in Brno, which doesn’t have much tourism (just a bunch of expats), but in Prague you’ll have no problem with English, I’m sure.