Installs the specified system locale or its portion as the new C locale.
The modifications remain in effect and influences the execution of all locale-sensitive C library functions until the next call to setlocale. If locale is a null pointer, setlocale queries the current C locale without modifying it.
locale category identifier, one of the GLib.LocaleCategory macros.
system-specific locale identifier. Can be "" for the user-preferred locale or "C" for the minimal locale
Pointer to a string identifying the C locale after applying the changes, if any, or null pointer on failure.