Niether knew them fleuntly, Deniro was comfortable with it, like he a little bit, Pacino only knew common phrases, so he had to kinda learn it, this was one of two reasons for the sicily scene with vitelli and he had fabrizio translate. One reason is that pacino wasnt that good with italian and that coppola doesnt really like the use of subtitles. And also another interesting fact, at the time of the movie Pacino did not know how to drive a car.