Category is a named constant (or string) specifying the category of the functions affected by the locale setting:
LC_ALL for all of the below
LC_COLLATE for string comparison, see strcoll()
LC_CTYPE for character classification and conversion, for example strtoupper()
LC_MONETARY for localeconv()
LC_NUMERIC for decimal separator (See also: localeconv())
LC_TIME for date and time formatting with strftime()
If locale is the empty string "", the locale names will be set from the values of environment variables with the same names as the above categories, or from "LANG".
If locale is zero or "0", the locale setting is not affected, only the current setting is returned.
Setlocale returns the new current locale, or FALSE if the locale functionality is not implemented in the platform, the specified locale does not exist or the category name is invalid. An invalid category name also causes a warning message.