LC_ALL, LC_COLLATE, LC_CTYPE, LC_MONETARY, LC_NUMERIC, LC_TIME

Each of the above macro constants expand to integer constant expressions with distinct values that are suitable for use as the first argument of setlocale.

Additional macro constants, with names that begin with LC_ followed by at least one uppercase letter, may be defined in locale.h. For example, the POSIX specification requires LC_MESSAGES (which controls, among other things, perror and strerror), ISO/IEC 30112:2014 (2014 draft) additionally defines LC_IDENTIFICATION, LC_XLITERATE, LC_NAME, LC_ADDRESS, LC_TELEPHONE, LC_PAPER, LC_MEASUREMENT, and LC_KEYBOARD, which are supported by the GNU C library (except for LC_XLITERATE)

Example

#include <stdio.h>
#include <locale.h>
#include <time.h>
#include <wchar.h>
 
int main(void)
{
    setlocale(LC_ALL, "en_US.UTF-8"); // the C locale will be the UTF-8 enabled English
    setlocale(LC_NUMERIC, "de_DE.utf8");   // decimal dot will be German
    setlocale(LC_TIME, "ja_JP.utf8");      // date/time formatting will be Japanese
    wchar_t str[100];
    time_t t = time(NULL);
    wcsftime(str, 100, L"%A %c", localtime(&t));
    wprintf(L"Number: %.2f\nDate: %Ls\n", 3.14, str);
}

Number: 3,14
Date: 月曜日 2017年09月25日 13時00分15秒

References