I like to think I'm a good listener, and can understand most people. That good ear is part of being a musician, isn't it?
Flying from Amsterdam to Belfast a couple of years ago, the flight attendant started the safety speech. Pure gobbledygook. She played with the life preserver, buckled the belt, did some disco poses with her arms, and then there was silence. "Right," i thought. "That was the Dutch, now it's English time." Then she started speaking Dutch and I realized that the entire first half was English with an Irish accent. I hadn't understood a word!
I assume with communication and world travel being so easy, the English language is going to become even more fragmented. Living in Toronto, where roughly 50% of the population is not Canadian-born, we all add our own cultural words into the otherwise-mostly-english conversation. And sometimes we pick up each others' terminology and start using it.