UK. Though maybe Cambridge is slow/conservative in noticing usage change. https://dictionary.cambridge.org/dictionary/english/corn

UK farmer here - since childhood in 1970s we've always called it "corn harvest" when talking about wheat, barley, oats, rye, etc.; sometimes Maize is referred to as corn-on-the-cob.

Perhaps it is not current usage, but I guess it might have be common English usage till 19th century. I think that I have seen corn used generically in novels and other works from before that time.

May be you can find reference to corn in Chaucer, I don't exactly remember if it is there for sure.

In German "Korn" means grain - e.g. "Vollkorn" meaning "whole grain".

John Barleycorn Must Die.

Yes, but this is recent, hundred years or so.

The English countryside is an exception. Townies are more likely to use it in the American sense though these days.