> Also for most of those things, you want to be explicit about when to use the locale and when to not.
Right. And that's where the POSIX C API falls down. The locale isn't named explicitly. Its not a function parameter. Its specified via a global variable that gets shared between all your threads.
You might think you can use scanf to parse a string in a JSON file. It might appear to work fine on your local computer. But scanf behaves differently depending on the system locale. You can wrap scanf with a helper function which sets the locale to something sensible, calls scanf, and restores the locale. But because the locale is shared with other threads, which might be depending on the locale in other ways. So this can introduce race conditions.
The whole thing is horribly designed - and it leads to buggy, unreliable code that is hard to reason about. Even in the best case, introducing thread syncronization into a function like sscanf will lead to a dramatic decrease in performance.
Also for most of those things, you want to be explicit about when to use the locale and when to not.