I suspect Commonwealth or Asia. Is your definition of sandwich cold things between sliced bread and burger hot things in a bun?

A piece of hot chicken between bread in Italy would likely be a panino, france a sandwich, spain a bocadillo, Portugal sandes, Japan a sando, mexico a torta, Argentina a sanguche.

I think you overestimate how many people use burger for things that don't refer to the American concept. A lot of cultures have hot sandwiches and thus (ham)burger is often distinctly the American concept of a ground beef patty. Where this breaks down outside of the Commonwealth is often from cultures without things in bread that got exposed to the generic burger via fast food chain terminology. Not surprising there.