You should definitely choose whichever is the most interesting for you, first and foremost! Unless you have any specific favourite, or one that you know holds lots of cultural references or metaphor or whatever. Or even better, whichever one you can get a hold of easily. Believe me - I am having HUGE problems getting my dissertation corpus!
I'm a bit confused about the whole "language not culture" part of you request though. Surely language and culture are inextricably intertwined?
Perhaps he needs to focus on the language that the author used in his script? That could be " language aspect of a movie"
We all know Woodie Allen is also a script writer, besides being a director and actor.
And a quite good writer..
[Edited at 2008-11-21 09:51 GMT]