I needed to set up a client's site with an English admin area, but a French display for theme information. (i.e. site display online is in French.)
I achieved this by:
–installing a theme with its own language files
–setting my wp-config.php file to expect french
–removing the french .mo and .po files from /wp-content/languages/ so the admin area would have no files to draw from and have to default to english.
It worked, but if feels a little... dishonest? :) I was just wondering if anyone had a more official idea about how to do this. Would love to know!