Your right DoZ, I'm British yet I couldn't understand what your example meant.
A place called banks a couple of miles up the road from where I live you may hear something like:
"Ef thee plantum craaaaaidly on that slawp, thee'll be fust en Bonks"
Which means, if you plant them nicely on that slope, they'll be first in Banks (Banks being the town)
That is a strong form of the dialect from Lancashire but as I live on the border between two different regions my dialect changes depending on how much time I'm spending in either region.
As for American slang, well I understand much of what's said but that's because most of what I hear is through films which are probably more considerate in using strong dialect.
I'm sure there's regions of the US (such as the south)that I'd find very hard to understand.
