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.