Fine, if he thinks they spoke in Greek, make a version in Aramaic and Greek. Big deal, most folks (in case this idiot didn't realise) don't really speak ancient Aramaic or Latin or Old Greek and that's a pretty big majority in the English speaking world.
I wonder if Gibson's choice of languages was for the Biblical era scholar community?
What is noteworthy is that Gibson did not film the movie in the English language, meaning that most of the world will have to suffer thru subtitles in order to understand the dialogue. And my understanding of Roman culture at the time of Christ is that many ''Romans'' were bi-lingual, speaking both Greek and Latin. I think that Gibson was aiming for a feel of authenticity by using Latin & Aramaic. As you pointed out, the ancient versions of Greek & Aramaic would almost certainly be unintelligible to modern speakers of the language anyway. Basically, I think Gibson took a big risk by not filming the movie in English. But then, he's on record as saying he didn't make the movie for commercial reasons.
All said & done, this article by Hitchens says more about Hitchens than either the film or Mel Gibson. And what it says is not pretty.