I've always been fascinated by them. I don't know if it's really true that the English language is the hardest to learn, but I can only imagine how difficult it must be to learn as a second (or third, fourth,...) language.
I once met an elderly Czech who was fluent in eight or nine languages. He said the one that gave him most difficulty was Hebrew. I'm a little in awe of people like that. I'm happy if I can keep all the ducks in a row in my own language.