Someone once told me (in the days before it became possible to easily check whether this might be an urban legend) that the Spanish pronounce the letter 'c' as a 'th' sound because there was once a King of Spain (Ferdinand?) who spoke with a lisp, and his subjects duly copied him. Had this been true, it would have been an example of a change in language caused by what linguists call prestige borrowing, although I don't know if the Spanish subjects would have copied the lisp for prestige, or because they expected the Spanish inquisition if they didn't.
The other day, listening to the radio, I was surprised to hear a Government spokesman (whose name unfortunately I missed) speaking about the future for nucular power in the U.K. I don't think this mispronunciation of 'nuclear' is common here, and it sounded quite awkward coming from an otherwise well-spoken Englishman. Was he trying to borrow some prestige from that great nuculariser George Dubya? Or perhaps he's a fan of Homer "You know boys, a nucular reactor is a lot like a woman" Simpson.
Who knows? As Homer said: "English? Who needs that? I'm never going to England!"