Windows 10 RS4 supports locale ".utf8" already, I wonder why `setlocale` doesn't make fmt::print with L-string work?

Now we can do this `std` way on Windows 10 v1803 or later
````
std::setlocale(LC_ALL, "en_US.utf8");
wprintf(L"zß水");
````
but `fmt::print` still do need this :
````
_setmode( _fileno(stdout), _O_WTEXT );
````
Otherwise, it only prints 'z'.

I wonder why?  Is there some performance hits to support `setlocale` like `wprintf`?