According to our multi-lingual map, in French, the Cathedral of St. Vitus is rendered as Cathedrale St-Guy. We have no idea, either.
By whatever name, it's exquisite, especially the modern stained glass. Just don't go up the tower if you have medical issues. Have a heart attack up there and you're a goner. There are almost three hundred steps in the spiral staircase, and on almost every one of them is a person going up and one going down. To help matters, the light switch is at arm level near the bottom, so the lights go out every minute or so.