US keyboards differ just enough from UK keyboards to throw me off slightly: " and @ changing places, in particular, are a problem to adjust to.
But French keyboards? As I learnt from several work trips to Paris offices: French keyboards are a nightmare to work on for developers used to QWERTY layouts. Most letters are where you'd expect them to be, but some aren't. Punctuation keys are scattered. Unshifted, the number keys produce punctuation and accents; you have to press Shift to get numbers.
So: when working overseas, always take a keyboard with you. But even then, sometimes fate and Microsoft conspire against you. What's harder to use than a French keyboard? A UK keyboard to which Windows has applied the French keyboard map. Try typing your password on that, sucker!