This might be slightly off-topic, and not all that important, please excuse me.
But I'm wondering, when you're writing software to be used worldwide, and your main focus is not America, but more European countries and countries previously colonized by England, is it better to use American spelling of words (Americanized vs. Americanised, color vs. colour ) or English spelling?
Sure most people won't even notice, but I do notice when an English website uses American spelling, even though it's a local (not American) website, and I'm wondering whether other people notice too. Is American spelling by now the accepted standard for a global website, or should I rather spell it the way I expect most of my users to be used to?
I don't think there is a data-driven, objectively true answer to this question. For me, it is to some extent a matter of personal preference.
What I would say is:
The American strength in popular culture has spread US English worldwide. In most situations it IMHO boils down to British English if your audience is in the UK; and American English for everywhere else.
Unless you're targeting just UK I would say use American English from personal experience.
When I used British English on my site not a month went by without someone pointing out a spelling "mistake" - but with US culture being dominant we (Brits) are more used to seeing US English. Americans tend to just see a mistake.
Proof? - search "adg" - http://successfulsoftware.net/2010/05/27/learning-lessons-from-13-failed-software-products/ (As a Brit it pains me to say this!)
This is completely testable with A/B testing and Google Analytics. Depending on how much copy needs to be rewritten, it may be worth testing.
British English is far more widespread than US English. The spelling conventions in Ireland, Australia, New Zealand, South Africa, and even Canada are closer to British than to US English. British spellings are used by the ISO (though, like Oxford University Press, they favour -ize over -ise ; this is perfectly correct, if slightly unusual, British English). To a large extent, British English is world English, especially in formal contexts. The USA is an outlier.
However, Ryan is correct to say that Americans are more insular than others, and less likely to be aware of such spelling differences. Isaac Asimov, a prolific writer and very well educated man, was surprised to see the word maths in British English, while I, a schoolboy at the time I read this essay by Asimov, knew well that Americans abbreviate mathematics to math.