Change Drupal interface language back to English
You can change the language back to English by adding this to the url of any page:

&amp;language=en


EDIT:

To have a seperate admin and front end language I'd suggest looking at the Admin Language module - http://drupal.org/project/admin_language

SECOND EDIT:

To do this via role have a look at this page - http://deglos.com/blog/2010/06/12/overriding-language-drupal-backend

They are doing what you want with a custom module. You could throw in the user role code there using the global $user.