Receiving "Undefined property" notice for
$langcode = $form_state['#' . $form_state['#entity_type']]->language;
on line 24 of button_field.module

I am using button field on a Drupal Commerce order summary admin page. Looks like the commerce order does not have a language. Or maybe it is because my website is configured to be only in one language. Anyway, looks like you should handle the case where the variable is not set.

Good luck