> I don’t know why so many languages have string functions that should take a locale but provide an overload that doesn’t and which uses the system locale as the default.

That‘s a relict from the past, before Unicode became prevalent, where systems used to ever only work in a single locale, and where users expected applications (running locally of course) to use the local system locale. Hence applying the system locale to everything was the standard behavior for applications. The C standard library was defined in that way, and since then every other runtime (usually based on C at some level) does the same.