#include <iostream>
#include <clocale>
#include <cwctype>
#include <cstdlib>

int main()
{
    std::setlocale(LC_ALL, "en_US.utf8");

    char utf8[] = {'\xc3', '\x81'};
    wchar_t big;
    std::mbtowc(&big, utf8, sizeof utf8);
// or just skip the whole utf8 conversion
//    wchar_t big = L'Á';

    wchar_t small = std::towlower(big);

    std::wcout << "Big: " << big  << '\n'
               << "Small: " << small << '\n';
}