Should a British developer spell functions with American English?

Depends on the team and company. If it is a US company or predominately US team then I'd say American English. If instead it is a British based company, or predominately a British team then I'd say British standards. As an American I would have no issues with using British spellings of words if that was the decided path.

In the end, I think consistency in the code base is most important and a note on the repository which says which it follows. It only becomes a problem if people use both interchangeably or inconsistently, that makes it a pain in the ass.

I used to do some gigs in the Middle East. Even within single documents you'd find the same words spelled in both the British English and American English spelling.

As a Canadian, only found the 's' instead of 'z' in words that end with -ize unfamiliar. The other flip-flopping didn't bother me because it was essentially what I always see here at home with Canadian/American spelling norms.

Usually there would be a standard dictating how things should look and feel, and that would indicate spelling norms. But when there isn't a standard, I'd just consider who I think would be the next people who have to look at my work, and go from there.

I always felt though, that the lack of standardizing (or standardising, as the case may be) the language of choice is the type of thing that leads to hard-to-find bugs. There should never be the option to have both a getColour() and getColor() defined.

Though I prefer British spelling just about all programming languages and APIs use American English so I use that when programming. I do not want to search code twice for analyze/analyse, color/colour etc.

who's reading the code?