Well Denny, the French phrase seems polite and even elegant, but I trust your primary goal here would be to find the darn toilet as quickly as possible, not to be either polite nor elegant. "Toiletta?" or just "Toilet please?" would meet your need, while speaking French would likely just confuse the matter and delay your quest. I do believe that more people in Italy will speak either English or maybe some German than will speak French. In some public places you may need to pay half a euro or so to use the WC, not usually true in museums. In small businesses, like restaurants and stores, the WC is often for customers only, which is why one of my mottoes in Italy is: "When you need a bathroom, go into a gelateria, order a gelato and pay for it, then ask to use the bathroom, then pick up your gelato on the way out. Then, enjoy the gelato!"