The translatable order-related information (the products, the types of taxes) are stored in the database using the language that the customer used to place the order. When you view an order in admin (or a customer views it via My Account->Order History), the headings for Billing Address, Shipping Address, Telephone, etc. are displayed in the admin/shop current language.
This puzzled me, too, and DrByte confirmed the behavior back in April: http://www.zen-cart.com/forum/showthread.php?t=153658.



