История изменений

     Only three locales are defined by default, the empty string «» which
     denotes the native environment, and the «C» and «POSIX» locales, which
     denote the C language environment.  A locale argument of NULL causes
     setlocale() to return the current locale.  By default, C programs start
     in the «C» locale.  The only function in the library that sets the locale
     is setlocale(); the locale is never changed as a side effect of some
     other routine.

DISCLAIMER: я не специалист в этой области, я просто умею хорошо находить информацию ☺

     Only three locales are defined by default, the empty string «» which
     denotes the native environment, and the «C» and «POSIX» locales, which
     denote the C language environment.  A locale argument of NULL causes
     setlocale() to return the current locale.  By default, C programs start
     in the «C» locale.  The only function in the library that sets the locale
     is setlocale(); the locale is never changed as a side effect of some
     other routine.