I create this simple program:

#include <iostream>
#include <locale>
#include <clocale>
#include <cstdio>

using namespace std;

int main()
{
    locale loc("ru_RU.UTF-8");
    cout.imbue(loc);
    cout << 1234.56 << std::endl;

    setlocale(LC_NUMERIC, "ru_RU.UTF-8");
    printf("%'.2f\n", 1234.56);

    return 0;
}


Result is:
1?234,56: (in hex): \x31\xC2\x32\x33\x34\x2C\x35\x36 
1 234,56: (in hex): \x31\x20\x32\x33\x34\x2C\x35\x36 
Here '?' is invalid symbol \xC2

then I try (code in C):
#include <stdio.h>
#include <locale.h>
int main() {
   setlocale(LC_ALL, "ru_RU.UTF-8");
   printf("\"%s\"\n", localeconv()->thousands_sep);
   printf("%f\n", 1234.56);
   return 0;
}

Output is: " " (one space in double quotes)