Why does changing the thread language and back fail for known folders?

I am trying to get language-specific text for known folders.

Thanks to the reply to my earlier post "How can I get column property names in different languages?" I use SetThreadUILanguage() to select the language and then try to get the locale name for a known folder.

My PC has English and Thai installed. English is the current PC language.

When changing the thread language, I find that for the property name, as per the previous post, IPropertyDescription::GetDisplayName() works perfectly.

However, getting property values is not working correctly.

A call to SHGetFileInfo() works after the first change of language, but not after the second.

A call to IPropertyStore::GetValue() only returns the PC language text.

There may be other methods to retrieve the text, but I haven't found any others.

I would like to use SHGetFileInfo(), or an alternative, to get the text in the thread's current language.

I attach a working console program that shows the problem.

#include <iostream>
#include <objbase.h>
#include <propsys.h>
#include <propkey.h>
#include <shellapi.h>
#include <Shlobj.h>


#pragma comment(lib, "Ole32.lib")          // Included libraries here to help build the test program.
#pragma comment(lib, "Propsys.lib")
#pragma comment(lib, "Shell32.lib")



void ShowPropName(const PROPERTYKEY& infoKey)
{
    IPropertyDescription   *pIDesc;
    WCHAR                  *pczPropName;

    if(PSGetPropertyDescription(infoKey, IID_PPV_ARGS(&pIDesc)) == S_OK)
    {
        pIDesc->GetDisplayName(&pczPropName);

        wprintf(L"Property Name <%s>\n", pczPropName);

        CoTaskMemFree(pczPropName);
        pIDesc->Release();
    }
}


void ShowItemNameFromFileInfo(REFKNOWNFOLDERID fidFolder)
{
    LPITEMIDLIST            pidlFolder;
    SHFILEINFO              infoFile;

    if(SHGetKnownFolderIDList(fidFolder, 0, NULL, &pidlFolder) == S_OK)
    {
        if(SHGetFileInfo((LPCTSTR)pidlFolder, 0, &infoFile, sizeof(infoFile), SHGFI_PIDL | SHGFI_DISPLAYNAME) != 0)
        {
            wprintf(L"Item Name <%s>\n", infoFile.szDisplayName);
        }

        ILFree(pidlFolder);
    }
}


void ShowItemNameFromPropStore(const PROPERTYKEY& infoKey, REFKNOWNFOLDERID fidFolder)
{
    LPITEMIDLIST            pidlFolder;
    IPropertyStore         *pIStore;
    PROPVARIANT             propValue;

    if(SHGetKnownFolderIDList(fidFolder, 0, NULL, &pidlFolder) == S_OK)
    {
        if(SHGetPropertyStoreFromIDList(pidlFolder, GPS_DEFAULT, IID_IPropertyStore, (void **)&pIStore) == S_OK)
        {
            PropVariantInit(&propValue);
    
            if(pIStore->GetValue(infoKey, &propValue) == S_OK)
            {
                wprintf(L"Item Name <%s>\n", propValue.bstrVal);

                PropVariantClear(&propValue);
            }

            if(pIStore != NULL)
            {
                pIStore->Release();
            }
        }
    }
}


int main()
{
    LANGID         nLangPC;
    LANGID         nLang2;
    
    CoInitialize(nullptr);

    nLang2  = 0x041E;
    nLangPC = GetThreadUILanguage();

    wprintf(L"\n------- 0x%04X ------------\n", nLang2);

    SetThreadUILanguage(nLang2);                                // Set language 2
    
    ShowPropName(PKEY_FileName);                                // Language 2 - Correct

    ShowItemNameFromFileInfo(FOLDERID_Music);                   // Language 2 - Correct
    ShowItemNameFromPropStore(PKEY_FileName, FOLDERID_Music);   // Language PC - Wrong

    wprintf(L"\n------- 0x%04X ------------\n", nLangPC);

    SetThreadUILanguage(nLangPC);                               // Set language PC

    ShowPropName(PKEY_FileName);                                // Language PC - Correct

    ShowItemNameFromFileInfo(FOLDERID_Music);                   // Language 2 - Wrong
    ShowItemNameFromPropStore(PKEY_FileName, FOLDERID_Music);   // Language PC - Correct


    CoUninitialize();

    return 0;
}

A file/directory name cannot be "localized". But Windows has a specific Known Folders API that you can use to determine a known folder's localized name in any supported/installed Windows language.

Once you get an IKnownFolder COM reference, you can use the IKnownFolder::GetFolderDefinition method to get its pszLocalizedName. It's usually in a form like this: @%SystemRoot%\system32\windows.storage.dll,-21790

From this string you can use the SHLoadIndirectString function which will automatically determine the localized value using the current thread's UI language.

Here is a complete sample code:

#include <windows.h>
#include <stdio.h>
#include <propkey.h>
#include <shlwapi.h>
#include <Shlobj.h>

#pragma comment(lib, "propsys.lib")
#pragma comment(lib, "shlwapi.lib")

void ShowPropName(REFPROPERTYKEY key)
{
    IPropertyDescription* desc;
    PSGetPropertyDescription(key, IID_PPV_ARGS(&desc));
    if (desc)
    {
        WCHAR* name;
        desc->GetDisplayName(&name);
        if (name)
        {
            wprintf(L"Property Name <%s>\n", name);
            CoTaskMemFree(name);
        }
        desc->Release();
    }
}

void ShowKnownFolderName(REFKNOWNFOLDERID id)
{
    IKnownFolderManager* mgr;
    CoCreateInstance(CLSID_KnownFolderManager, nullptr, CLSCTX_ALL, IID_PPV_ARGS(&mgr));
    if (mgr)
    {
        IKnownFolder* folder;
        mgr->GetFolder(id, &folder);
        if (folder)
        {
            KNOWNFOLDER_DEFINITION def;
            if (SUCCEEDED(folder->GetFolderDefinition(&def)) && def.pszLocalizedName)
            {
                WCHAR localizedName[256];
                SHLoadIndirectString(def.pszLocalizedName, localizedName, _countof(localizedName), nullptr);
                wprintf(L"Folder Name <%s>\n", localizedName);
            }
            folder->Release();
        }
        mgr->Release();
    }
}

int main()
{
    LANGID nLangPC;
    LANGID nLang2;

    CoInitialize(nullptr);

    nLang2 = 0x040C; // french
    nLangPC = GetThreadUILanguage();

    wprintf(L"\n------- 0x%04X ------------\n", nLang2);

    SetThreadUILanguage(nLang2);
    ShowPropName(PKEY_FileName);
    ShowKnownFolderName(FOLDERID_Music);

    wprintf(L"\n------- 0x%04X ------------\n", nLangPC);

    SetThreadUILanguage(nLangPC);
    ShowPropName(PKEY_FileName);
    ShowKnownFolderName(FOLDERID_Music);

    CoUninitialize();
    return 0;
}