I didn't propose any solution or claim Shift-JIS fixes this. It doesn't really since a single Shift-JIS document can only encode the japanese variant, not both.

I didn't propose any solution or claim Shift-JIS fixes this. It doesn't really since a single Shift-JIS document can only encode the japanese variant, not both. However, a unicode codepoint which acted as a "language hint" switch would solve this, and wouldn't require doubling the number of han codepoints. There already are unicode variant selectors, but iiuc they only apply to a single character at a time, and no one actually uses them, so they're not very useful.