[ EDIT 09/23/2019 ] : I did finally trained a French Bidirectional Language Model with the MultiFiT configuration. It overpasses my other LMs (see my post) 
Thanks @jcatanza and @jolackner. I agree with you: this MultiFiT configuration (see last page of the paper) must be tested and should give better results than the ULMFiT one.
If I found time, I will (by the way, I’ve already published my results using the ULMFiT configuration for a French LM and Sentiment Classifier).