scheme@(guile-user)> (setlocale LC_ALL "does-not-exist")  
"C"



list gdt 8 ~ > cat l.c
#include <langinfo.h>
#include <nl_types.h>
#include <locale.h>

#include <stdio.h>
#include <assert.h>


int
main (int argc, char *argv[])
{
  char *s;

  s = setlocale (LC_ALL, "C");
  assert (s != NULL);

  printf ("DAY_1: %s\n", nl_langinfo (DAY_1));

  s = setlocale (LC_ALL, "en_GB");
  assert (s != NULL);

  printf ("DAY_1: %s\n", nl_langinfo (DAY_1));

  return 0;
}
list gdt 9 ~ > cc -o l l.c
list gdt 10 ~ > ./l
DAY_1: Sun
DAY_1: Sun



But if I run this on a linux box I get

gdt 78 ~ > ./l
DAY_1: Sunday
l: l.c:20: main: Assertion `s != ((void *)0)' failed.
Aborted


I'm afraid I don't really understand i18n details.