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)
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