Use French when no translation
Sorry for my small responsiveness. I see there are reactions on the 2 other languages. For Nourmande, I see there are no active users (only maintenance) and only a few editions. I remarked an anonymous Nourmande contributor who contributed yesterday, I left him/her a message on his/her talk page in the hope s/he will answer there or here. If there is no answer in the next month (or before if you want to do the merge), I’m personally in favor of the merge – and if someone in the future has arguments for other fallbacks, it can always be changed.