After quite a bit of testing we came up with the following solution:
in /classes/order/OrderHistory.php
we added
$topic = $result['osname'];
$data = array(
'{lastname}' => $result['lastname'],
'{firstname}' => $result['firstname'],
'{orderdate}' => Tools::displayDate($order->date_add),
'{id_order}' => (int)$this->id_order,
'{order_name}' => $order->getUniqReference()
);
(lines 424-431), in the e-mail template we use {orderdate} ..
;-)