main.cc

#include <algorithm>
#include <codecvt>
#include <iostream>
#include <locale>
#include <wchar.h>

int main()
{
    std::wstring test = L"łabądź";

    std::transform(test.begin(), test.end(), test.begin(), [&](wchar_t c)
            {
                switch(c)
                {
                    case L'ł':
                        return L'l';
                        break;
                    case L'ą':
                        return L'a';
                        break;
                    case L'ź':
                        return L'z';
                        break;
                    default:
                        return c;
                        break;
                }
            });

    std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t> converter;
    std::string converted_test = converter.to_bytes(test);

    std::cout << converted_test << "\n";

    return 0;
}