Having different languages for different sites. Is it possible?

I am running WordPress 3.0 Multisite with an english install and consequently english admin panels for all my sites.
Now I would like one of these admin panels (and the WordPress strings on site) to be in French. However if I simply add the French translation in wp-content/languages and modify wp.config.php, then everything is in English.

Is there a solution to that problem?

Thanks!