In reply to Onions: I suspect that the reason you haven't got many responses is that there isn't much multi-pitch climbing in northern France for the simple reason that there aren't any proper mountains.
I climbed at Clecy in Normandy once, there are some two pitch routes there but it was so scrappy that we didn't bother doing very much. There is probably some multi-pitch trad climbing on the Brittany coast, but really if you want to do multi-pitch climbing in France you are much better off heading to the Alps or further south.
Freyr might fit the bill (it's not in France but it's only a short distance from the border) - however I believe that the easier routes tend to be very polished.