It looks like a conflict is causing the WPML language parameter to be added before bbPress adds its /edit/
endpoint.
I suppose it’s possible inside bbPress to try and handle that differently, but I’m reluctant to make it too fancy since it might break something else.
Without really looking, my guess is that WPML should filter the permalink at a later priority (as late as possible) so that all other filters have a chance to hook in normally first.
Hi John, thanks for the hint. I switched the WPML system to use subdomains system with http://fr.mysite/forum/ for the french version, and it seems to work better with bbPress.