Daniel-
So, IMO, a language must exist independent of Wikipedia and not be hindered by a claim of copyright before an encyclopedia is started in it.
Well, this is another issue:
http://www.tokipona.org/ (C) Toki Pona is the intellectual property of Sonja Elen Kisa, 2001-2004. You may use these materials freely, as long as you do not compete with this site.
Given that Sonja is also the most active contributor to the Toki Pona Wikipedia that's probably not much of a big deal, though. Still, the "as long as you do not compete" clause is annoying.
Yes, I agree that conlangs are the more serious problem than natural languages. However, I don't think that there should be no criteria at all for natural languages. The three criteria that Andre proposed - ISO 639-2, more than 50 archived documents, or more than 10,000 speakers - seem reasonable, and would probably kick out most obscure conlangs, while leaving in legitimate spoken tongues, and dead languages too, if there's a written record of them (not that I care at all about those, but in the interest of wikipeace ..).
Regards,
Erik