ahahah if in doubt just wave your hands and shout "PIZZA!" they will be able to guess what you want :) But if you want to be polite you can say "Mi dai un pezzo di pizza per favore?". I used to work in a multi-cultural laboratory, my rule was that every day I had to learn a new word in another language, it was fun especially when when in turn others wanted to learn italian, often I had to teach some chinese students how to sing Italian Opera, it was hilarious! :)