Here is a solution I think works for Winforms:

// get cultures for a specific resource info
public static IEnumerable<CultureInfo> EnumSatelliteLanguages(string baseName)
{
    if (baseName == null)
        throw new ArgumentNullException("baseName");

    ResourceManager manager = new ResourceManager(baseName, Assembly.GetExecutingAssembly());
    ResourceSet set = manager.GetResourceSet(CultureInfo.InvariantCulture, true, false);
    if (set != null)
        yield return CultureInfo.InvariantCulture;

    foreach (CultureInfo culture in EnumSatelliteLanguages())
    {
        set = manager.GetResourceSet(culture, true, false);
        if (set != null)
            yield return culture;
    }
}

// determine what assemblies are available
public static IEnumerable<CultureInfo> EnumSatelliteLanguages()
{
    foreach (string directory in Directory.GetDirectories(AppDomain.CurrentDomain.BaseDirectory))
    {
        string name = Path.GetFileNameWithoutExtension(directory); // resource dir don't have an extension...

        // format is XX or XX-something, we discard directories that can't match.
        // could/should be replaced by a regex but we still need to catch cultures errors...
        if (name.Length < 2)
            continue;

        if (name.Length > 2 && name[2] != '-')
            continue;

        CultureInfo culture = null;
        try
        {
            culture = CultureInfo.GetCultureInfo(name);
        }
        catch
        {
            // not a good directory...
            continue;
        }

        string resName = Path.GetFileNameWithoutExtension(Assembly.GetExecutingAssembly().Location) + ".resources.dll";
        if (File.Exists(Path.Combine(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, name), resName)))
            yield return culture;
    }
}

And here is how you would use it for a WindowsFormsApplication1:

    List<CultureInfo> cultures = new List<CultureInfo>(EnumSatelliteLanguages("WindowsFormsApplication1.GUILanguage"));