If you think that editing gets complex as you have to keep track of individual “house styles” versus APA or MLA Style, versus Chicago Style, and so on — imagine adding another layer to the complexity as you need to keep track of Canadian English versus American English, versus British and even Australian.
You may not be dealing with differences in pronunciation, since you’re editing words on the page, but there are plenty of alternate spellings to keep track of, and even alternate words for the same thing.
Most people are aware of the “-our” words in Canada and Britain (flavour, saviour, behaviour, and so on) compared to the American versions (flavor, savior, behavior). But there are many more differences than that.
For example, think of the American lyrics, “I’ve been working on the railroad, all the livelong day.” Do you realize that in Canada you’d technically need to sing, “I’ve been working on the railway”? And as a Canadian, I remember driving in the U.S. and asking a gas station attendant if I could use the “washroom,” and getting a blank look. In most places I’ve visited in that country, they don’t use that word, but say it right out: “bathroom.” If I’d been in Britain, I might have had to use “toilet,” and in Australia, “comfort station.”
This is why, if you’re going to be editing materials from other English-speaking countries — which is more and more likely in the internet age — it’s very important to use the right dictionaries. You may have to amass quite a collection. And since even dictionaries from the same country will vary to some degree, you’ll want to establish ahead of time, with your client, which one is going to be the standard for the project.
The same thing will likely apply to style guides, and you’ll need to investigate whether, say, the Chicago Manual could properly be applied to a manuscript you receive from Australia. If your client wants you to conform to a guide that reflects Australian style, you’ll need to get some sort of access to that guide.
We may all be able to read each other’s books and other writings, because we all still do speak the same language. But these regional and national differences in grammar and spelling (and even, in some cases, punctuation) are very real. And we’ll ignore them at our peril.