I can easily see a way around it.
The same method as has been sometimes suggested as a regulated workflow on the monolingual arbcoms, and is defacto happening at least on the english wikipedia arbcom, in a fairly granularized manner.
Not have every arbcom member available for each case.
It might work on the basis of having a dedicated French language section of the arbcom, a Dutch language section of the arbcom, a Portuguese language section of the arbcom and so forth, with co-mingling between arbitrators from different language teams being an exception rather than a rule.
Or it might be that every arbcom case would work on a pick and mix principle, with arbitrators specifically announcing they are available for a case, either being able to understand the working language, or bringing in an assistant to help them understand the issues as they pertain to things said in the working language.
The fact that this would entail bringing in a fairly large pool of arbitrators, to me is a plus than a negative, as it would naturally decrease the workload on any one arbitrator.
If you're doing that, you might as well just give the cases to randomly chosen local arbcoms. It's an idea to consider, but I think a true meta-arbcom would be better.