Text: The subject of the e-mail is a resource string. You can find the key value in the ~\CMSResources\CMS.resx file. In this case it should be "ordernotification.customersubject" and "ordernotification.administratorsubject".
You can change the values for these key directly in the resource file. The "{0}" is a macro and the orderID is displayed instead.
Other way is to add these keys to the Site Manager -> Development -> UI Cultures -> click to edit En culture -> New string. Add the key value and the text you want to have. This will ensure that you will be able to easily translate the subjects in other languages.