GetUserPreferredUILanguages() never returns more than two languages

Question

I'm trying to retrieve the complete list of the user's preferred languages from a C++/Qt application, as configured in the "Region & language" page in the user's preferences:

For that, I am trying with the WinAPI function GetUserPreferredUILanguages(), on an up-to-date Windows 10 Pro system.

However, the function always only returns the first entry (the main Windows display language), and "en-US". If English is configured as the main language, then only "en-US" is returned. E.g., if I have (German, French, English) configured, ["de-de", "en-US"] is returned, French is omitted. If I add more languages to the list, they are omitted as well. I also looked at User Interface Language Management, but to no avail. GetSystemPreferredUILanguages() for example only returns "en-US". GetUILanguageFallbackList() returns ["de-de", "de", "en-US", "en"].

The code I use:

// calling GetUserPreferredUILanguages() twice, once to get number of 
// languages and required buffer size, then to get the actual data

ULONG numberOfLanguages = 0;
DWORD bufferLength = 0;
const auto result1 = GetUserPreferredUILanguages(MUI_LANGUAGE_NAME,
                                                 &numberOfLanguages,
                                                 nullptr,
                                                 &bufferLength);
// result1 is true, numberOfLanguages=2

QVector<wchar_t> languagesBuffer(static_cast<int>(bufferLength));
const auto result2 = GetUserPreferredUILanguages(MUI_LANGUAGE_NAME,
                                                 &numberOfLanguages,
                                                 languagesBuffer.data(),
                                                 &bufferLength);

// result2 is true, languageBuffer contains "de-de", "en-US"

Is this not the right function to use, or am I misunderstanding something about the language configuration in Windows 10? How can I get the complete list of preferred languages? I see UWP API that might do the job, but if possible, I'd like to use C API, as it integrated more easily with the C++ codebase at hand. (unmanaged C++, that is)

Answer 1

GlobalizationPreferences.Languages is usable from unmanaged C++ because GlobalizationPreferences has DualApiPartitionAttribute. Here is a C++/WinRT example of using GlobalizationPreferences.Languages:

#pragma once
#include <winrt/Windows.Foundation.Collections.h>
#include <winrt/Windows.System.UserProfile.h>
#include <iostream>
#pragma comment(lib, "windowsapp")

using namespace winrt;
using namespace Windows::Foundation;
using namespace Windows::System::UserProfile;

int main()
{
    winrt::init_apartment();

    for (const auto& lang : GlobalizationPreferences::Languages()) {
        std::wcout << lang.c_str() << std::endl;
    }
}

And a WRL example for those who cannot migrate to C++ 17:

#include <roapi.h>
#include <wrl.h>
#include <Windows.System.UserProfile.h>
#include <iostream>
#include <stdint.h>
#pragma comment(lib, "runtimeobject.lib")

using namespace Microsoft::WRL;
using namespace Microsoft::WRL::Wrappers;
using namespace ABI::Windows::Foundation::Collections;
using namespace ABI::Windows::System::UserProfile;

int main()
{
    RoInitializeWrapper initialize(RO_INIT_MULTITHREADED);
    if (FAILED(initialize)) {
        std::cerr << "RoInitialize failed" << std::endl;
        return 1;
    }

    ComPtr<IGlobalizationPreferencesStatics> gps;
    HRESULT hr = RoGetActivationFactory(
        HStringReference(
            RuntimeClass_Windows_System_UserProfile_GlobalizationPreferences)
            .Get(),
        IID_PPV_ARGS(&gps));
    if (FAILED(hr)) {
        std::cerr << "RoGetActivationFactory failed" << std::endl;
        return 1;
    }

    ComPtr<IVectorView<HSTRING>> langs;
    hr = gps->get_Languages(&langs);
    if (FAILED(hr)) {
        std::cerr << "Could not get Languages" << std::endl;
        return 1;
    }

    uint32_t size;
    hr = langs->get_Size(&size);
    if (FAILED(hr)) {
        std::cerr << "Could not get Size" << std::endl;
        return 1;
    }
    for (uint32_t i = 0; i < size; ++i) {
        HString lang;
        hr = langs->GetAt(i, lang.GetAddressOf());
        if (FAILED(hr)) {
            std::cerr << "Could not get Languages[" << i << "]" << std::endl;
            continue;
        }
        std::wcout << lang.GetRawBuffer(nullptr) << std::endl;
    }
}