The override of joomla doesn't work in emails.
Instead, try to add the code:
$language =& JFactory::getLanguage();
$language->load('com_users', JPATH_SITE, $language->getTag(), true);
before the code:
if($model) $result = $model->activate($activation);
in the file components/com_hikashop/controllers/checkout.php
that should load the translations of com_users before sending the email to the admin.
If you want to add a message to tell the user that an admin will have to validate his account, it's in the same function that you can do that, like that:
$app->enqueueMessage('An administrator will now have to validate the activation of your account.');