designer.Visible = true;
var host = (IDesignerHost)designer.Object;
var provider = TypeDescriptor.GetProvider(host.RootComponent);
var typeDescriptor = provider.GetExtendedTypeDescriptor(host.RootComponent);
if (typeDescriptor == null)
    continue;

var propCollection = typeDescriptor.GetProperties();
var propDesc = propCollection["Localizable"];
if (propDesc != null && host.RootComponent != null &&
    (bool?)propDesc.GetValue(host.RootComponent) != true)
{
    try
    {
        propDesc.SetValue(host.RootComponent, true);
    }
    catch (Exception ex)
    {
        // log the error
    }

    // save changes
}