There is no question that a Scottish accent is the most difficult accent to understand.
"Introduced" the Scottish accent via Youtube to some Mexican co-workers who were fluent English speakers and they freaked out. Will never forget one saying "WTF is this?"
It's odd because Dutch itself, being a Germanic language, has a lot of pronunciation commonality with the most conservative of all English accents, Scottish. You can hear it in this video, where a sentence composed of common Germanic words is spoken in every Germanic language:
If it weren't for Trainspotting, which I adore, I'd be utterly useless at understanding Scots accents. As it is, I can just barely manage.
Honestly, I'm regularly surprised by how nice and patient people from everywhere are, paying attention to my words even if my English is not, even today, as good as I would like it to be. Including French people. Including British people.