Must admit, I've always wondered about the Japanese use of the title, since most of their history the Japanese haven't bothered foreign countries at all, let alone trying to rule them.
The Japanese don't use that title, it's simply the word the English-speaking world uses for the Japanese monarch. The Japanese word translates as something like
Divine Sovereign, I believe. I suppose somebody figured "king" wasn't fancy enough for a ruler whose subjects considered him to be a divine being.
In English we use the word and concept of "Emperor" for the rulers of people who never used that term or even one that translates to our concept of what an emperor is. The Romans didn't even use it. Their word for what we call their emperors was
Princeps, which means "first"; as they were trying to maintain the fiction, at least for a while, that they still lived in a republic. From there we get our word "prince."
Imperator, which is where our word "emperor" comes from, simply meant "commander."
Language can be a funny business.