It's hard to be more explicit that it is considered both.
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Added in edit
In speaking with a French colleague, he says that "inférieur" often means "less-than-or-equal-to" rather than "strictly-less-than", so the passage you quote would still imply that 0 is negative (and most likely also positive).
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Second edit:
> In France, "positive" means "supérieur à 0", and "supérieur à " means "greater than or equal to". Similarly, "négative" means "inférieur à 0", that is "less than or equal to 0".
> (We have the similar reaction towards the anglosaxon world and the introduction of nonnegative…)
That would cause all kinds of problems, so I'd be pretty surprised if it turned out to be true.
I note that this is the heading of the relevant wikipedia page:
> Un nombre négatif est un nombre réel qui est inférieur à zéro, comme −3 ou −π.
( https://fr.wikipedia.org/wiki/Nombre_n%C3%A9gatif )
It'd be hard to be more explicit that zéro is not a negative number.