use a .mo file, if you need the pot file feel free to download at the following url :
http://blog.ouetquand.org/wp-inst/wp-includes/languages/muwordpress.pot
Then use poEdit to translate into your language, if you need the french one mail me fred [at] ouetquand [dot] org
Thanks so much for the pronpt answer and your help!
Is this a translation file for both Wp and WPMU?