Hardly any fullbacks are strong with both feet.
Hes more likely to become a more effective fullback and play for france by improving key modern fullback attributes ie anticipation, positioning, marking, tackling, dribbling, of the ball movement, technical ability, crossing etc
Being strong with both feet isnt really a requirement... many world class players are totally one footed, and there are many two footed players who never rise above amateur levels.
Many fans get a bit up in arms around 'supposed to be professionals, they should be able to use both feet'. Reality is, theres limited time in a day. If you want to reach/maximise your potential, time is better served on key attributes for the position you play in, rather than spending load of time bringing your weak foot up to a simikar standard.
Even with tens, hundreds of hours time investment... it will always be weaker than the foot youve used since 2 years old. As a result, Its usually more effective to adapt game to alway using stronget foot... way you position yourself, carry the ball, balance, distribute the ball etc... and accuracy success over long term will be much higher, than using even fairly strong, but still weaker foot

.
There is of course going to be rhe odd situation where it perhaps would raise the question. But imo its isolated, and not worth investment over spending time on other stuff.
I guess the question to ask is... if its as much of a crucial, no brainer skill to hsve as some fans allude to... why are professional football coaches in elite level sport not alotting more time to it, given all the training they do... and that weaker foot training is hardly physically intensive, rhat could lead to overtraining/burnout...