I worked on a job once where there were Irish ,Jamaicans,an English speaking Frenchman and god knows what. The only two I had difficulty understanding were a pair o' Scots . Terribly nice they were , but nobody could understand them. People were a bit embarrassed by having to ask them to repeat everything ,but they took a sort of pride in the fact that they were uninteligible to the sassanach .
I caught them out once when they asked me if I could understand what "we twa hae puddled in the burn frae morning sun til dine" meant.