I've read somewhere that Potlach was a kind of celebration of northwestern Indian tribes. A chief invited representatives of other groups and gave them presents. "Potlach" means "give".
"Potluck" was supposed to be derived from this. But your explanations sounds logic too, so who knows... There are a lot of popular errors, maybe this is one.
It may even be that "pot luck" was translated into French and found its way into your language that way. <img src="/ubbthreads/images/graemlins/think.gif" alt="" />
