CredUIPromptForWindowsCredentials return junk characters if you P/Invoke it from managed code on OS’s such as Spanish or French.

[DllImport("credui", EntryPoint = "CredUnPackAuthenticationBufferW", CharSet = CharSet.Unicode, SetLastError = true)]

internal static extern bool CredUnPackAuthenticationBuffer(

   uint dwFlags,

    IntPtr pAuthBuffer,

    uint cbAuthBuffer,

    [Out] IntPtr pszUserName,

    ref uint pcchMaxUserName,

    [Out] IntPtr pszDomainName,

    ref uint pcchMaxDomainName,

    [Out] IntPtr pszPassword,

    ref uint pcchMaxPassword);

….

uint maxPassword = CREDUI_MAX_PASSWORD_LENGTH;

IntPtr passwordBuffer = Marshal.AllocCoTaskMem((int)maxPassword);

uint maxLogonName = CREDUI_MAX_USERNAME_LENGTH;

IntPtr logonNameBuffer = Marshal.AllocCoTaskMem((int)maxLogonName);

uint maxDomainNameHint = CREDUI_MAX_USERNAME_LENGTH;

IntPtr domainNameHintBuffer = Marshal.AllocCoTaskMem((int)maxDomainNameHint);

…..

if(CredUnPackAuthenticationBuffer( CRED_PACK_PROTECTED_CREDENTIALS,  

                                   outBuffer,

                                   outBufferSize,

                                   logonNameBuffer,

                                   ref maxLogonName,

                                   domainNameHintBuffer,

                                   ref maxDomainNameHint,

                                   passwordBuffer,

                                   ref maxPassword))

{

    String logonName = Marshal.PtrToStringUni(logonNameBuffer, (int)maxLogonName);

    Console.WriteLine("User Name: {0}", logonName);

}

else

{

    throw new Win32Exception(Marshal.GetLastWin32Error());

}