Returns a BCP-47 language tag (e.g. "en-US", "uk-UA", "jv-Java") for the given HKL (keyboard layout handle).

GetLanguageTagFromHKL.cpp

#include <windows.h>
#include <string>
#include <vector>

/*
 * Returns a BCP-47 language tag (e.g. "en-US", "uk-UA", "jv-Java")
 * for the given HKL (keyboard layout handle).
 *
 * WM_INPUTLANGCHANGE provides HKL, whose low word is LANGID. However,
 * for transient keyboard layouts (LOCALE_TRANSIENT_KEYBOARD*), LANGID
 * does not map correctly via LCID APIs.
 *
 * In such cases, the function resolves the tag via:
 * HKCU\Control Panel\International\User Profile
 * matching TransientLangId → language tag.
 *
 * Falls back to LCIDToLocaleName for regular LANGIDs.
 *
 * This matches the approach used in WinForms and recommended by Microsoft.
 */
std::wstring GetLanguageTagFromHKL(HKL hkl)
{
    LANGID langId = LOWORD((ULONG_PTR)hkl);

    if (langId == LOCALE_TRANSIENT_KEYBOARD1 ||
        langId == LOCALE_TRANSIENT_KEYBOARD2 ||
        langId == LOCALE_TRANSIENT_KEYBOARD3 ||
        langId == LOCALE_TRANSIENT_KEYBOARD4)
    {
        HKEY key;
        if (RegOpenKeyExW(HKEY_CURRENT_USER,
                          L"Control Panel\\International\\User Profile",
                          0, KEY_READ, &key) == ERROR_SUCCESS)
        {
            DWORD size = 0;
            if (RegGetValueW(key, nullptr, L"Languages",
                             RRF_RT_REG_MULTI_SZ, nullptr, nullptr, &size) == ERROR_SUCCESS)
            {
                std::vector<wchar_t> buf(size / sizeof(wchar_t));

                if (RegGetValueW(key, nullptr, L"Languages",
                                 RRF_RT_REG_MULTI_SZ, nullptr,
                                 buf.data(), &size) == ERROR_SUCCESS)
                {
                    for (const wchar_t* p = buf.data(); *p; p += wcslen(p) + 1)
                    {
                        HKEY sub = nullptr;
                        DWORD val, cb = sizeof(val);

                        if (RegOpenKeyExW(key, p, 0, KEY_READ, &sub) == ERROR_SUCCESS &&
                            RegGetValueW(sub, nullptr, L"TransientLangId",
                                         RRF_RT_REG_DWORD, nullptr,
                                         &val, &cb) == ERROR_SUCCESS &&
                            (LANGID)val == langId)
                        {
                            RegCloseKey(sub);
                            RegCloseKey(key);
                            return p;
                        }

                        if (sub) RegCloseKey(sub);
                    }
                }
            }
            RegCloseKey(key);
        }
    }

    wchar_t locale[LOCALE_NAME_MAX_LENGTH];
    return LCIDToLocaleName(MAKELCID(langId, SORT_DEFAULT),
                            locale,
                            ARRAYSIZE(locale),
                            0)
           ? locale
           : L"";
}

case WM_INPUTLANGCHANGE:
{
    HKL hkl = (HKL)lParam;

    std::wstring tag = GetLanguageTagFromHKL(hkl);
    // → "en-US", "uk-UA", "jv-Java", etc.
}
break;