Foreign kings' names

[Posteritatis]

43. When in doubt I try to go by what's officially used in other languages

If Japan referred to Akihito as Tenno in English-language statements, for instance, then I'd use that title. As it is, however, the Imperial Household Agency uses the term "Emperor" in English, and so that's that for me. That general pattern holds for most of the titles you're thinking about - we call the Spanish monarch "King Juan Carlos" because that's how the Spanish refer to him when speaking English. With that sort of thing the would-be-translatee is the final arbiter - especially at that level, where diplomatic niceties are going to make it effectively mandatory anyway.

If I have no indication at all I enjoy trying to learn and use (or at least just know) the local names and titles. I run across lots of translated names historically (I'm a Roman history geek) and have always found going by the 'true names' does a lot to humanize societies that are a bit more off the mental radar. It's a fun challenge in particular when digging really far back in history, and tends to humanize some of the guys we only ever really knew of in terms of a few dusty names.