I can say this much: intense category theory makes not very much less sense delivered in a foreign language.
But seriously, I actually had a lot of fun trying to keep up, and I understood more than I expected. The blackboard-writing served as a useful Rosetta stone, allowing me to see familiar mathematical turns of phrase, and hear how they are rendered in French.
The seminar went on until like 4:30(!), and then Noam and I talked for a good while after, and then walked over to Cité Universitaire for a bit of a picnic with some other friends of his. I was feeling pretty solidly not-introvert-tired on the walk over, but the weather was just a bit too cold for me to actually relax, and I ducked out after not too long.