The AF web site for bike travel states that you must declare it a couple of days before, and it is supposed to be a 50€ or so upcharge.
Now... I did travel with Air France on my way to Kona last october (Paris - San Francisco and back), and I did not declare anything. And I was not charged with anything. I had a Scicon hard case and it went to oversized luggage without any issues. The guy at the counter asked some questions if it was an electric bike as batteries need to be removed, but that was all.