Duke of Dust, you don't say where you got your rail fare info, but the size of the reservation supplement suggests to me that you looked it up on raileurope.com. They charge a large premium above what you would pay by buying the ticket in Italy.

The easiest way to deal with the language barrier is to write the information on a piece of paper (use Italian place names - e.g. Firenze vs. Florence) to help your pidgin Italian be more easily understood. Did this in Rome while making sleeper reservations to Venice and it worked well. Don't forget that to Europeans, 6/5 is May 6 and not June 5.

Buying the ticket locally works best if you do it a day or so early - so you won't be worried about having enough time.