YESSTR and NOSTR selector to nl_langinfo don't have locale data for POSIX and
en_US (and possibly others, these are just the two that happen to be important
for me).  Many locales do have these strings, which seems to indicate that it's
just missing data rather than the feature not being supported.  I'll attach a
patch that adds these strings.

Reproducer:

$ cat y.c
#define _GNU_SOURCE
#include <locale.h>
#include <langinfo.h>
#include <stdio.h>

int main (void) {
  setlocale (LC_ALL, "");
  printf ("|%s|%s|\n", nl_langinfo (YESEXPR), nl_langinfo (YESSTR));
}

$ gcc y.c -o y
$ LANG=C ./y
|^[yY]||
$ LANG=en_US ./y
|^[yY].*||