Most of our customers are US users, so that is why it is set up that way. In addition, we are a US based company. In addition, to get your ideas seen, you would be best to post in the wishlist forum, where the product development team will see it and determine whether or not we will incorporate it into a future version of the product.
We try to help our customers as quickly as possible and the response times are within our support policies which is part of the user agreement. Also, changing the date format isn't actually a supported functionality, so I'm actually giving you additional instruction outside of what is required.
Try this instead:
$mailHeader .= "Date: ". date('d-m-Y H:i:s').$lineEnd;