>Hi everybody,
>
>I've checked this link http://grammarist.com/spelling/cancel/ but I am not 100% sure which spelling should I choose. The software is written by US company, but may be used by US, Canadian, NZ and Australian clients. It may also be used internationally in other countries.

It is already used internationally. Check your dictionaries. You have really gone native :).

>So, just wondering what spelling do you prefer in your code?

I spell it "cancelled" (even though FF marks it as an error) whenever I remember to. Likewise, it was seen in several anglophone countries, and never had a complaint. I think people in these countries are generally used to bad spelling and correct most of the stuff in their minds without even noticing what they just did. So just don't worry.