How do you turn on Canadian English spelling in Pages?

Try this

Open a blank Document in Pages

From Menu

Go to File

> Advanced

> Language and Region

Document Language and Region

Select English (Again if Necessary)

You should see options for Region beginning with Australia, select Canada

In Pages, you may wish to force Canadian English in the Edit menu : Spelling and Grammar : Show Spelling and Grammar : Automatic by Language selection, rather than trust it will do the right thing, and select Canadian for you.