Jakob Nielsen, a Danish national, operates under the misapprehension that there are exactly two ways to spell in English – British and American.
If your site is based in a single, English-speaking country and you don’t mind being viewed as a local site from that country, use its language variant. So a U.S. site should use American English, whereas a U.K. site should use British English. Similarly, sites based in Australia or other Commonwealth countries that predominantly use British English should use that variant.
Canada has its own set of spellings and that set doesn’t match any of the U.S., U.K., or “Commonwealth” spellings.
Canadian sites that mainly target the U.S. should use American English, unless they want to emphasize the fact that they’re foreign. (This can be a selling point, but most American users view it negatively.)
He has no research basis backing that up, of course.
Nielsen’s posting does what his postings usually do – gives no firm advice and leads into an ad for a seminar his company is running. It also mixes up the issues of spelling and word choice. Fundamentally, the posting reiterates the lie that there is such a thing as an international English. There isn’t – not in speech and not in writing. (Don’t believe me? Phone an Indian call centre.)
After ten years of continuous error correction, why do Nielsen and his clients insist on believing he never makes mistakes? Why is it impossible to report mistakes? Some of us actually pay people to error-check our work.