But.. you already posted a solution to your problem - use chinese font for chinese and japanese font for japanese. There is no problem. I mean, you would also presumably use a western font for latin and another separate font for cyrillic, since japanese fonts universally have absolutely dreadful kerning on latin (and often omit cyrillic entirely).
Having to know metadata about text is precisely what made the pre-unicode days so bad. If you're writing a word document, sure, choose your fonts, but if you're rendering a web page that doesn't happen to declare its language, things aren't so simple. (And if you're writing software meant to correctly handle user input in multiple languages, good luck...)
