Italy, like the rest of Europe and many other countries around the world, uses the International signage system established by the 1968 Vienna U.N. Convention on Road Signs and Signals.
http://en.wikipedia.org/wiki/Vienna_Convention_on_Road_Signs_and_Signals
The US has chosen not to adopt that International standard (and what do you expect from the only country that instead of adopting the easier metric system still measures distances in feet and rods) and instead has decided to use signs written in English language, rather than symbols, such as "No U turn" or "No Parking" or "right lane must turn right". Obviously the US has decided to do so to force everybody to learn English when driving in the US. The fact that I've never seen a road sign in America that is in both English and Spanish is a clear conspiracy against Mexican immigrants.
However, regarding the accusation that in Italy signs are only in Italian, the City of Florence has installed bilingual ZTL signs in both Italian and English (see link below) with even a red and green light to indicate when the ZTL is active or when access is permitted (generally at night).
The signs in Italian and English, plus the green/red lights, have been installed gradually since 2009. But apparently that is still not preventing some Americans from getting fines for entering the ZTL. I'm not exactly sure at this point what the City of Florence can do to help these people. How about not checking in their brain along with their luggage when they travel?
http://www.firenzedabere.it/images/recensioni2/pannello_luminoso_ztl_firenze_2009.jpg