Air France and Northwest are both members of SkyTeam, and if you are top tier elite on either airline you get free access to their lounges when flying internationally.

I haven't tried upgrading with WorldPerks miles on AirFrance flights yet, but if you're using Northwest WorldPerks miles you might be restricted to being able to upgrade only tickets in the more expensive Y or B buckets to business class (if upgrade space is available). And those flights between Paris & Bangkok would have to be purchased on Northwest ticket stock for the WorldPerks people to be able to access the records to upgrade them. You can't upgrade to (or get free seats in) First Class on AirFrance with WorldPerks miles.

If you know you're going to be doing a lot of flying on AirFrance, why not enroll in FlyingBlue (or stick with it, if you're enrolled already)?