[1] shadowm@nanacore:~/src% cat locale_test.cpp
#include <iostream>
#include <clocale>

using namespace std;

const char* try_set_locale(const char* locale)
{
        locale = setlocale(LC_ALL, locale);
        if (!locale)
        {
                locale = "<invalid>";
        }
        return locale;
}

int main(int, char**)
{
        cout << ".UTF-8:    " << try_set_locale(".UTF-8") << endl;
        cout << "<default>: " << try_set_locale("") << endl;

        return 0;
}

[1] shadowm@nanacore:~/src% g++ -W -Wall locale_test.cpp -o locale_test && ./locale_test
.UTF-8:    <invalid>
<default>: en_US.UTF-8
[1] shadowm@nanacore:~/src%