Even for websites in English, there are a number of pitfalls I've stumbled on.
1) The word "billion" means 1,000 million in the US, but 1,000,000 million in the UK and Europe.
2) Numerical date formats are different, with much of the world using date/month/year but the US style being month/date/year. Very confusing for the first 12 days of every month!
3) The US style is to separate every three digits of a large number with commas, but some countries alternate commas and periods. I still don't have this one straight how does the alternating style show where a true decimal starts?
4) From what I've heard, it is highly offensive in Japan to offer a radio button that is already checked by default. It's relatively offensive to me, too, but lots of American sites do it all the time.
What other tricky international areas have people noticed?