wint_t towlower_l(wint_t c,
                  locale_t loc);

towlower_l converts an uppercase letter to a corresponding lowercase letter in locale loc.

If the argument c is a wide character for which iswupper_l is true and there are one or more corresponding wide characters, in the locale loc, for which iswlower_l is true, towlower_l returns one (and always the same one for any given locale) of the corresponding wide characters; otherwise, c is returned unchanged.