> If you're really hardcore processing multilingual text you need an equivalent to language tags anyway, because you need a dictionary for word-wrapping, date formatting, quote marks etc along with changing fonts (or glyph selectors in the same font.) TeX has them too.
Sure. And once you're doing that you don't gain a lot of benefit from unicode AFAICS, because you have to track these spans of locale-specific text.
Lets you copy characters that are the same (like people's names) between text though.
People's names are the thing you should be most careful to not do that with! Most Japanese people will generally put up with seeing the Chinese rendering of a character in ordinary text, but they really don't want you to do that to their names!
But usually people only care about their language, so it goes by the system UI language and it works out.