Have a look at Edit > Spelling & Grammar > Show Spelling & Grammar (cmd-
In the dialogue that appears, there’s a dropdown box to change the language. By default it’s set to automatic, which picks up your system language (I think), but you can force it to any language you choose.
And I think for an even more persistent setting, the “Text” tab within the Keyboard System Preference pane is where you select which spelling dictionary all software should use by default. So if you prefer a particular variant of English, setting it there should do the trick.