Parent post says
> spans with their own encoding.
so yes it is about pages with multiple encodings. A span's smaller than a page!
The Unicode answer is "variation selectors" which are used for some historical variant kanji, but not for whole language switching. I suppose they could be used for that too though.
In my reply here, I can type in english and russian at once. Привет, мир.
Yet, if I try to type chinese on one line, and japanese on the next, I cannot do it. Hacker news does not let me enter "lang" tags, so I can only type either the chinese or the japanese variant of a kanji.