A lot would depend on the specific exit type as well. Passengers sitting at a self-help exit mid-cabin will not be able to see any cabin crew signals - plus cabin crew will be busy assessing their own door situations.
To be honest, I don't believe there will ever be a perfect combination of language capabilities among the crew and passengers. You just have to be vigilant to ensure that you have the right people at the exits for the conditions on the day. If you ever done a SAGA or Lourdes charter, you'd know what I mean.
I can see why AF would want to ensure a common language exists between the crew and the passengers at the exits and if it happens to be French, then so be it. Doesn't mean I wouldn't find it frustrating though.