You are totally right, I completly forgot to check the content encoding of posts wich is UTF-8, forcing to ISO will just mess up all post, that solution is not the good way to go.

Yeah, could you try this one :
http://slyserv.dyndns.org/osm/French-utf8.zip

If punbb is not doing complex conversion from it’s template string, or if not computing too much string length not knowing it is UTF-8, then I hope the language pack provided before could do.