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.