the animus translates; in assassins creed 2, rebecca tells desmond that there are some gitches, and some italian words wouldn''t translate, and french would not translate at all, so desmond does hear some italian words, but not enough to fluently speak it.

Desmond also does a ridcukousy over the top italian accent in the game, which rebecca cakls him a racist for, but yeah, if he spoke italian due to the bleeding effect, he probably wouldn't understand what he is saying. I have seen the bleeding effect theory in lots of fanfic, however. But I think ubisofts official line on languages, is that it is translation software. I could be wrong.