Perhaps you need GetUserDefaultUILanguage. The system's settings and user settings may not be the same.
User Interface Language Management
int wmain()
{
wcout << "GetUserDefaultUILanguage: " << GetUserDefaultUILanguage() << "\n";
wcout << "GetSystemDefaultUILanguage: " << GetSystemDefaultUILanguage() << "\n";
wcout << "\n";
wcout << "GetUserDefaultLangID: " << GetUserDefaultLangID() << "\n";
wcout << "GetSystemDefaultLangID: " << GetSystemDefaultLangID() << "\n";
wcout << "\n";
wcout << "GetUserDefaultLCID: " << GetUserDefaultLCID() << "\n";
wcout << "GetSystemDefaultLCID: " << GetSystemDefaultLCID() << "\n";
wcout << "\n";
wchar_t buf[100];
LCID lcid = GetUserDefaultLCID();
cout << "GetUserDefaultLCID: " << "\n";
if (GetLocaleInfo(lcid, LOCALE_ILANGUAGE, buf, 100)) wcout << buf << "\n";
if (GetLocaleInfo(lcid, LOCALE_SENGLANGUAGE, buf, 100)) wcout << buf << "\n";
if (GetLocaleInfo(lcid, LOCALE_SISO639LANGNAME, buf, 100)) wcout << buf << "\n";
return 0;
}