You can choose to display the OrderBox interface, as well as the notification mails to your Customers and Sub-Resellers in any of several different languages.
To specify this language preference
Login to your Reseller Control Panel. See details
Point to Settings -> Other Branding Settings-> and then click Default Language.
Select the radio button next to the desired language.
This will change the language on the Control Panels as well as any notification mails sent out by the system to your Customers and Sub-Resellers. However, your Customers and Sub-Resellers also have the option to set their language preference for themselves. This would over-ride your specified preference, for that particular Customer or Sub-Reseller.