Hi Marc,
I'm an American, a bit of a Francophile, whose not in BA at the moment. Here's some advice for a French guy:
-There's a bar called Merval on Viamonte in the Microcentro. It's a lot like the bar Footsie (or FTSE?) that was on Rue de l'Opera in Paris, in that there is kind of stock market for drinks. There's baby foot (foosball). Pretty fun. There's lots of French people in there, as well as the local ladies who love them (wink, wink). What's nice about it is has a bit of an anglo bar/pub feel in terms of talking with people, so good combo.
-OMG, I wish I'd been French or Italian in BA, though HeyBA did manage to maximize his yanqui charisma. The girls will adore you, just don't try too hard, and politely evaluate the scene with a calm gallic air of analysis. Abortion is illegal (still, right? or is that wrong?), and there's a fair amount of women that wouldn't mind their baby daddy having euros or dollars. They're easy to spot after you've been there awhile. Take precautions. Watch out for some "falling in love" type stuff from more honest BA girls who are really infatuated with the idea of themselves being European enough to land a European guy. If you feel like "you've never been treated like this before," you probably won't be treated like that for very long.