That should be checked and acknowledged by a least a few local community leads. Maybe ask in another specific thread here? (I think other communities said they would like that, and I have seen some messaging saying that the call this year would be open up up until september… it seems quite impractical –why open early if you only tell people in september ?– but the community collective can choose! )
We ask the languages spoken. Do you mean that if someone is speaking only flemish or french, you will not select them? Maybe you could ask other multilingual communities (ie. the swiss community) for advices here?
If you plan to only accept participants that speaks english, we indeed need to be very clear on that criteria.
You can check the usual criterias here: http://www.museomix.org/appel-a-participants-2015/