I think in that case the idea would either be that you should then have an array of strings, each of which may have its own language set, or that the string should be labelled as "containing Latin and Cyrillic", but still not able to include arbitrary other characters from Unicode. And multi-lingual text still generally breaks on words... Kilobytes of Latin text with a single Cyrillic character in the middle of a word is very suspicious, in a way that kilobytes of Latin text with a single Cyrillic word isn't.

Of course you'd always need an "unrestricted" string (to speak to the rest of the system if necessary), but there are very few natural strings out there in the world that consist of half-a-dozen languages just mishmashed together. Those exceptions can be treated as exceptions.