The worst part is that there is no way to turn off the locale sensitivity.

This is bad for not only performance, but correctness. If you are writing a JSON serializer and you use sprintf() to format float/double values, someone can make you produce incorrect output by setting the locale to something that uses "," as a decimal separator.

And there is no way to turn this off!

This is one moment where the difference between an application language and a system language seems clear. An application language could maybe assume that string formatting is for showing to a user. A systems language should assume that you are implementing protocols, and that mere user preference should not change your output.