Hi Micky, I think there seems to be a bit of confusion here.
From your comment above
I'm using a french WP 3.8.1 installation with "responsive" theme 188.8.131.52
and c'ant get the plugin ( 3.9.4 ) back end working in french.
That is not possible at the present as stated by @Ivy2120 in the following comment
@laCabriole, At the moment the back-end is only in English. We are working on making it so the plugin will translate into your language.
Then @mra13 mentioned the following
Hi Micky, We do have the French translation in the plugin for the shopping cart (front end).
That means that only the front end is translated in French not the back end.
Let us know if this make sense to you.