I was finally able to change from US English spelling to Canadian spelling, but my biggest problem is with grammar. I have tried to enable a grammar tool, but can't find one I trust. It would be wonderful if the darn program included a grammar tool of its own. Heck, even Word does that.
ProWriting Aid works with Scrivener. I've used the grammar software mostly on MS Word and like it a lot. I have yet to try it on Scrivener because, at the moment, my Scrivener projects are not in English.