I don't watch a lot of films because most of them are formulaic rubbish! However one film I enjoyed greatly is Waking Life. The entire dialogue is in the form of conversation - and the content is largely philosophical. It would be better suited to the advanced learner, but it certainly presents some commonly used slang.
You know what I'm saying? Do you understand what I am talking about?
Here is a (short) clip from the film where the subject of language is considered.
YouTube - Waking Life - Language
- For Teachers