[OJFord]

That's not the case though, just as, more obviously, 'Mrs' isn't limited in meaning to 'this is a female adult'.

Increasingly and to the point of arguable totality, such titles are bestowed by popularity, committee, and HM Government. Damned shame, since it would mean more what it should were it not given to celebrity riff-raff for 'services to sport'.

I digress. You say it's fine as a non-'UK royalist' to use an improper title; I say I bet most of the world doesn't use even Mr, and I'd do my utmost to pay proper respect to local custom and any honours.

[pseudalopex]

Does local custom work both ways? I think most Americans would consider "Mister" perfectly courteous.

[OJFord]

I'm not sure what you mean by working both ways, I'm saying as the speaker one respects the subject's title, even if not 'recognised' natively.

For example, I'm not Catholic, but I would of course refer to Pope Francis; not 'Mr Francis', or anything involving his birth (as opposed to regnal) name.

[pseudalopex]

I think the norm is the current surroundings, not wherever the subject is from. Dr. Smith is Smith-sensei in Japan. The New York Times is known for its use of courtesy titles, and it calls knights "Mr.".

Pope is a high office, and Francis is his chosen name. It would be rude of you to call him by his birth name; it would be rude of him to insist you call him "Your Holiness", although you probably should if you're visiting the Vatican.