mangue: The rules changed awhile back (maybe a year or so ago) - before that, horizontal lines were allowed - but a certain pattern by white made it very easy for white to win - so the rules were changed.
If the French rules still say horizontal is allowed, then I would guess it is a bug that the rules in that language was never updated.
You may want to post it to the bug tracker that the rules are not correct in French. (since I'm not sure how often Fencer reads this board)
White seems to have a pretty big advantage. Do you think maybe a simple rule of "can not move your piece back to the same spot on the next move - you have to at least wait 1 move before moving the same piece back to the same spot" would help?
And that is you can not move B3 to B4, then on your very next move you can not move B4 to B3. You can move B1 to B3, or move B4 somewhere else - but not back to the same spot.