Can/should I speak Japanese in Taiwan and S. Korea?

First, an example for context: I live in Europe, and a surprising number of people working in restaurants and stores in other countries sometimes prefer speaking German over English. I’ll point to a specific experience I had in an Alsace wine shop where no one spoke English, but they definitely spoke German.

(It’s not remotely universal, to be fair, but the German tourism industry is strooong.)

I read on a random post today that someone had a good experience in S. Korea speaking Japanese. That made me wonder whether the above phenomenon (let’s call it a “Vacation lingua franca?”) might exist in E Asia. I speak neither Korean nor Taiwan’s native language, so I thought it’d be helpful to have a fallback in case English didn’t work. (Although I’m aware that relations are pretty sour atm.)