On Posix, you need the nl_langinfo() and nl_langinfo_l() functions from <langinfo.h>.  These functions are documented at <http://pubs.opengroup.org/onlinepubs/9699919799/functions/nl_langinfo.html>, and the constants d at <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/langinfo.h.html>.

On Windows, you need to call EnumCalendarInfoExEx if you have dropped support for Vista and earlier versions, or if not, then follow the links from the page about it.  The function is documented at <https://docs.microsoft.com/en-us/windows/desktop/api/Winnls/nf-winnls-enumcalendarinfoexex>, and the constants that specify particular pieces of information at <https://docs.microsoft.com/en-us/windows/desktop/Intl/calendar-type-information>.  (I have never used these interfaces myself.)

I hope this is helpful.