I am not sure I quite agree. The local arbitrators on say the Portuguese wikipedia might not have been chosen for their familiarity with minor languages in the (former and current) Portuguese colonies, just as an example, which a putative meta arbcom team with a working language of Portuguese, might quite easily be.
I guess that all depends on what responsibilities the meta-arbcom would have, which I don't think has been decided on. If it's primarily arbitrating disputes (as the name would suggest), the skills needed are much the same regardless of the nature of the dispute.
I don't quite see how a pure english language meta-arbcom would be truer.
It wouldn't be an English Language arbcom, it would an arbcom that uses English as a lingua franca. That's the only way to allow people from different languages to work together to resolve issues, which I think would be a good feature of a central arbcom.