Hi Tessa,
I’m not sure how you would accomplish that and it’s a quite unusual request, though it is probably possible. Perhaps you could ensure that WP_LANG is not defined until as late as possible, and then define it using your own code according to whether the request is admin/front-end in nature?
Alternatively, some browsers offer excellent near-real time translation built in which you could investigate.
I don’t think this is one we can really help too much with here on the forum, though. Sorry – and good luck!