> For example on an android phone, set language to Japanese and all kanji is in a Japanese font by default.

If it's an app that uses the native UI toolkit, sure. If it's using one of the package-it-up frameworks, you'd better hope the developer configured it correctly.

> For more niche uses you can usually set the font or language on a per app basis.

You very often can't, or it's impractically difficult for regular users. Try changing your locale but not your language and watch how many programs screw it up.