On my Blogger dashboard I get a lot of choice of languages, but the one I want isn't there. I can choose English (United Kingdom) or English (United States), but neither is the way I spell things; there's no English (Canada). The same is true most places where I can choose a user interface language - websites, browsers, word processors, and so on.
Canadian English spelling is a multiple personality disorder in text form. It's a mix of both British and American spelling. Usually we use -our instead of -or for "honour" and "neighbour" and usually -ction instead of -xion for "connection". Sometimes we use -ise or -yse for "criticise" or "analyse" and sometimes we use -ize or -yze. The only rule that covers everything is "one or the other, but not both." If you pick "honour" you must also use "neighbour" and if you use "criticize" you must also use "analyze".
Once upon a time there were good reasons for supplying only a limited set of options. Computer memory and speed were much more limited; it would have been easy to justify omitting a complex option for either reason or both. But now that my browser offers me a half-dozen alternative words when I make a spelling mistake, that's no longer a good excuse.
It's not enough to provide an "additional dictionary" into which to put my "misspelled" words. If I select American spelling and put "neighbour" into my personal dictionary, then "neighbor" is a spelling mistake -- and so is "honor" even if I've never used it yet. The converse applies if I select British spelling as the default.
There's a mechanism for telling software what I want: I can specify a "locale" of the form
For simple situations that would be "en_US" or "en_UK" (without a variant); "en_CA" could be some default (likely -our -ize -ction) with variants such as "-ise-xtion" for selecting the others. There aren't very many choices, and the combinations can be automatically generated from a slightly-annotated base version.
I imagine my Scots cousins would like "en_UK_Scots" variant, too, since some of them (the Home Rule supporters and the separatists) insist that Scots is a different language from English.