> start-of-language and end-of-language markers

Unicode used to have language tagging but they've been (mostly) deprecated:

https://en.wikipedia.org/wiki/Tags_(Unicode_block)

https://www.unicode.org/reports/tr7/tr7-1.html

The lack of such markers prevents Unicode from encoding strings of mixed Japanese and Chinese text correctly. Or in the case of a piece of software that must accept both Chinese and Japanese names for different people, Unicode is insufficient for encoding the written forms of the names.