chillerdragon@ChillersMacBookPro cplay % cat robster.c 
/*
    File: robster.c
*/
#include <iostream>
#include <locale>
using namespace std;
int main()
{
    cout << "With unset locale:" << endl;
    cout << "  setlocale(LC_ALL, NULL): " << setlocale(LC_ALL, NULL) << endl;
    cout << "  std::locale(\"\").name(): " << std::locale("").name() << endl;
    cout << "With setlocale(LC_ALL, \"\"):" << endl;
    setlocale(LC_ALL, "");
    cout << "  setlocale(LC_ALL, NULL): " << setlocale(LC_ALL, NULL) << endl;
    cout << "  std::locale(\"\").name(): " << std::locale("").name() << endl;
    return 0;
}
chillerdragon@ChillersMacBookPro cplay % g++ robster.c 
clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
chillerdragon@ChillersMacBookPro cplay % ./a.out 
With unset locale:
  setlocale(LC_ALL, NULL): C
  std::locale("").name(): 
With setlocale(LC_ALL, ""):
  setlocale(LC_ALL, NULL): en_US.UTF-8
  std::locale("").name(): 
chillerdragon@ChillersMacBookPro cplay % locale
LANG="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_CTYPE="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_ALL="en_US.UTF-8"
chillerdragon@ChillersMacBookPro cplay % uname -a
Darwin ChillersMacBookPro.fritz.box 21.6.0 Darwin Kernel Version 21.6.0: Mon Dec 19 20:44:01 PST 2022; root:xnu-8020.240.18~2/RELEASE_X86_64 x86_64
chillerdragon@ChillersMacBookPro cplay %