We travelled from Rome with Celebrity a few years ago - on Galaxy. I think because we are English they put us on a table with Canadians - very nice people. Everyone else seemed to be English or American. There was no Italian spoken anywhere that I remember! Incidentally we did our own flights and booked a few days in Rome each end of the cruise. It worked brilliantly.
As far as the announcements go it's really down to how many Italians/German etc. are actually on board. Celebrity normally attract a strong US/UK contingent so at most expect English/Italian over the P.A. Entertainment normally remains US/UK.
As Wilba says the announcements will be made in English, there are often other nationalities on board and major announcements will be made in their language as well but it will depend on the numbers from these countries each cruise as to which language is used. Fortunately Celebrity and Azamara keep their announcements to a minimum and seldom do they intrude on your cabin area unlike RCI who like to call you to every event as often as they see fit. I'm sure you will enjoy her she seems like a great cruise ship, (if you like them large) ...Neil.