What is the Windows equivalent for en_US.UTF-8 locale?

Basically, you are out of luck: http://www.siao2.com/2007/01/03/1392379.aspx

Although there isn't good support for named locales, Visual Studio 2010 does include the UTF-8 conversion facets required by C++11: std::codecvt_utf8 for UCS2 and std::codecvt_utf8_utf16 for UTF-16:

#include <fstream>
#include <iostream>
#include <string>
#include <locale>
#include <codecvt>
void prepare_file()
{
    // UTF-8 data
    char utf8[] = {'\x7a',                       // latin small letter 'z' U+007a
                   '\xe6','\xb0','\xb4',         // CJK ideograph "water"  U+6c34
                   '\xf0','\x9d','\x84','\x8b'}; // musical sign segno U+1d10b
    std::ofstream fout("text.txt");
    fout.write(utf8, sizeof utf8);
}
void test_file_utf16()
{
    std::wifstream fin("text.txt");
    fin.imbue(std::locale(fin.getloc(), new std::codecvt_utf8_utf16<wchar_t>));
    std::cout << "Read from file using UTF-8/UTF-16 codecvt\n";
    for(wchar_t c; fin >> c; )
        std::cout << std::hex << std::showbase << c << '\n';
}
void test_file_ucs2()
{
    std::wifstream fin("text.txt");
    fin.imbue(std::locale(fin.getloc(), new std::codecvt_utf8<wchar_t>));
    std::cout << "Read from file using UTF-8/UCS2 codecvt\n";
    for(wchar_t c; fin >> c; )
        std::cout << std::hex << std::showbase << c << '\n';
}
int main()
{
    prepare_file();
    test_file_utf16();
    test_file_ucs2();
}

this outputs, on my Visual Studio 2010 EE SP1

Read from file using UTF-8/UTF-16 codecvt
0x7a
0x6c34
0xd834
0xdd0b
Read from file using UTF-8/UCS2 codecvt
0x7a
0x6c34
0xd10b
Press any key to continue . . .