Sets the current locale of the program.
locale is a string specifying a locale;
category is an optional string describing which category to change:
the default category is
The function returns the name of the new locale,
or nil if the request cannot be honored.
locale is the empty string,
the current locale is set to an implementation-defined native locale.
locale is the string "
the current locale is set to the standard C locale.
When called with nil as the first argument,
this function only returns the name of the current locale
for the given category.