The Wikipedia article says that the spoken languages are Aramaic and Latin. Are these roughly 50/50, or does one predominate? I could watch this and try to determine it for myself, but I'm not confident that I can even tell the difference between the two.
What I know of the story itself hints that Latin is probably only spoken by Pontus Pilate, and maybe the Roman soldiers at the end of the film. But given that it seems Gibson took some artistic liberties with it, this might not be the case.
Mostly Aramaic, if this is to be believed:
When an Arabic speaking Muslim friend saw Mel Gibson’s movie Passion of the Christ with most of the dialogue in Aramaic, he was very surprised that he did not need most of the subtitles in English to understand the movie!
According to Mark Goodacre's academic blog:
one of the ballrooms hosts an event entitled "An Interview with the Writers of the Passion." The writers in question were Benedict Fitzgerald, co-screenwriter, and William J. Fulco, S.J., the film's theological consultant, who was responsible for the translations to Aramaic and Latin.
[...]
David Shepherd did put the difficult questions, though, and in particular pressed Fulco on the issue of Greek. Why Latin and Aramaic? Why not Greek? Fulco explained that they had tried using Greek too, but that it did not work and they thought they would have lots of complaints about pronunciation. He said that the advantage of using Latin and Aramaic was the sound of these two languages -- they could have the actors speaking variously in Aramaic and Italianate Latin and then every viewer would be able to distinguish between who was talking in which language because of the basic sounds.
This article explains a reason for the language choice:
The task of achieving linguistic authenticity fell to Rev. William Fulco, a Jesuit priest and professor of ancient Mediterranean studies at Loyola Marymount University in Los Angeles.
[...]
Other linguistic tricks of Fulco's serve a function in the script.
For example, he incorporated deliberate dialogue errors in the scenes where the Roman soldiers, speaking Aramaic, are shouting to Jewish crowds, who respond in Latin. To illustrate the groups' inability to communicate with each other, each side speaks with incorrect pronunciations and word endings. Later, "there's an exchange where Pilate addresses Jesus in Aramaic, and Jesus answers in Latin. It's kind of a nifty little symbolic thing: Jesus is going to beat him at his own game," Fulco said.
