[Plugin WP Super Cache] Modifying .htaccess for querystrings

I’ve been researching this for awhile now, I’m working on this site:
http://ywamburtigny.com

and it’s running very slow so i’m rocking out the wp-super-cache. the site displays in multiple languages (click the swiss flag in the top right corner) via appending a get query string to the url (?lang=fr). i’ve tried to modify the .htaccess a bit, but i’m often getting half french half english pages due to the way the cache is working.

basically, i’m wondering if there is any way to modify .htaccess so that it puts a different page in the cache depending on the query string.. so for example ywamburtigny.com is stored in the cache separately from ywamburtigny.com/?lang=fr.

otherwise, the second best option would be to cache the english version only, and ignore anything with a querystring as far as supercache goes by modifying .htaccess but everything i’ve found online to do that doesn’t seem to be working properly. i’m working off the default .htaccess right now.

thanks a ton
phil