Try Glaswegian. I'm from the south of England and on occasion hitch-hiked in Scotland. Got a lift from Aberdeen where they speak perfectly understandable English to Glasgow with a Glaswegian building contractor. I don't think I understood more than half of what he said along the way.
We once had an employee from Westinghouse from the Netherlands over at our house for Thanksgiving. The poor man could understand everybody in the family except for my grandfather. He finally asked us what he was doing wrong. We all laughed and simply pointed out that the problem was my grandfather's accent, not his English and that a significant portion of America wouldn't be able to understand my grandfather, either.