When I was in school we were treated to a movie version of Shakespeare’s MacBeth. Early on it became obvious that no one in the class (this was Los Angeles) could understand anything that was being said. Personally, I found that it didn’t take too long to get into the rhythm of the dialogue and it became easier and easier to understand. Scottish dialogue is just that way.
Have you ever heard Sean Connery speak in his natural Scottish brogue?
I have read a few books written in Scottish dialogue with much the same experience: start out slow and before you know it, even the most difficult dialogue makes sense. But when I stop to think about it, I am sounding out the words in my head as if they are dialogue in a movie. I wonder if my lips move.

Right now I am reading Filth by Irvine Welsh. I best reveal right off that in Scottish English or in the Scots language, swearing is both imaginative and ubiquitous. Or is it just the author? I suspect the language is reasonably accurate for the class of people Welsh is including in his novel, whether it’s youthful drug users in Trainspotting or hard-nosed policemen in Filth.
I remember going down to AFIPS when I was drafted in the 1960’s and hanging out with a group of young men applying to Officers Candidate School. For the most part these were friendly, articulate, well-educated midwestern men but as soon as they were in a room with future Army types, the vocabulary declined into a continuous stream of fuck. If you’ve ever been in the Army or dabbled in the drug culture, you probably won’t have any problem with the colorful language in Welsh’s novel.
That is, if you are open to new idioms and alternate vocabulary referencing all the usual sources of profanity, like bodily functions and parts.
I was interested in reading Filth now because, despite being considered unfilmable, they actually made a movie from the novel. I’m about half-way through the novel and although it is a little rough, a movie seems entirely possible. One thing about adaptations is that they generally have to modify or eliminate parts of the book to maintain the cinematic vision (and to keep the movie from becoming long and boring). For instance, I don’t think the movie needs to emphasize the anti-hero’s anal eczema: he can scratch and swear and look uncomfortable with much more presentable prickly-heat on his chest.
So far, if you enjoy sausage rolls, recreational drugs, underage trim, and overly graphic head-busting, then Filth is for you. Perhaps you can get an idea about the book from the trailer for the movie: