Ideally, the document format should specify which parts are in what language. Both MS Word and HTML can do that.
But this is "off-band" information and it kind of defeats the purpose of unicode. Moreover it is extremely annoying in practice: imagine that you write a text (e.g., a comment on HN) where you want to explain the character differences between japanese and chinese. How do you get to do that? There are no "language selector characters" in unicode!
The Variation Selectors Unicode block¹ has three variation selectors designated for this purpose:
• U+FE00 VARIATION SELECTOR-1
• U+FE01 VARIATION SELECTOR-2
• U+FE02 VARIATION SELECTOR-3
I assume they work like U+FE0E VARIATION SELECTOR-15 (for 'text presentation' of the preceding character) and U+FE0F VARIATION SELECTOR-16 (for 'emoji presentation' of the preceding character). You can see examples of that in action here².
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¹ — https://en.wikipedia.org/wiki/Variation_Selectors_%28Unicode...
² — https://apple.stackexchange.com/questions/180400/how-to-turn...
EDIT: just to try, it seems that it works 平仮名 ひらがな 漢字 汉字 한자
I guess the accusations of Unicode being somewhat centered on western languages do have a point.
The original motivation for the efforts that resulted in Han unification was to help with library and bibliography management (some of these efforts were by non-CJK speakers). One of the original design goals of Unicode was to be able to represent all of the characters in existing character sets uniquely (so two distinct characters in some charset requires two distinct characters in Unicode), and another design goal was to be able to facilitate conversion between different character sets representing the same script. This dovetailed nicely with the existing efforts to unify CJK scripts for bibliographies, hence Han unification.