It looks to me as though this is covered on page 4 of the English rules:
As long as generals of one power are stacked, a player
can transfer troops between them whenever he desires. He
can do it even during the action stage of another player.
Troops can never be transferred between generals which are
not stacked or which belong to different powers.
A general who loses his last troop is removed from the
board, unless the general is in a stack and it is possible to
transfer at least one troop to him. In such a case transfer is
mandatory: a general never is removed from the board as
long as he has one troop or as long as one troop can be transferred to him.
I interpret this to mean that it is up to the French player to choose which stacked general has how many troops.