Retranslate: No - if we have a case from a project whose participants use French as a second lanugage, simply pick arbitrators who speak french.
If you select a subset of the committee based on languages spoken, it's harder to ensure a "fair trial", since it's not a random selection. It also removes the benefit of having various points of view - the French speaking members are likely to be the members active on the French projects, so you lose the wealth of knowledge that members of other projects could bring.
Even worse: Since in this case we are talking about a conflict that is impossible to resolve in French wp, having arbitrators who are possibly active contriburors or admins in French wp just means that they may be a side of the conflict and it is unlikely that they remain neutral.
Cheers Yaroslav