Thats probably impossible. The longest ship canal i know of is the white sea baltic, at 227 km, the Canal des deux Mers is some 430 km, about twice as long.
The St Lawrence seaway is longer, but i dont know if the CANAL portion is anywhere near that long. I think not.
Ship canals are very expensive, and building them purely for prestige isnt going to happen.
The Panama canal cuts thousands and thousands of km, AND a really nasty sea passage out of manytrips. Its worth it. The Russians want to be able to get stuff from one coast to the other, and even those locks only take smallish ships.
You'd have to have the Straights of Gibraltar blocked to French vessels regularly for such an unprecedented canal to even be considered.