On Thu, 22 Dec 2005 15:07:05 -0500, Jörg Rhiemeier  
<[log in to unmask]> wrote:

> I'd say that a conlang is a language deliberately designed by
> an individual or a (small) group; a natlang is a language that
> evolved from another language during centuries of usage by a
> community.

This overlooks the pidgin/creole situation, where a complete language can  
emerge more or less fully formed in a matter of a couple of generations  
over a reasonably small (depending on your domain) group of people,  
without much if any planning.

Also, there are natlangs that consist of a very diverse set of dialects  
that are deliberately engineered, codified and koinized. Koine being the  
obvious example, but if my brain isn't playing tricks on me, I seem to  
recall Bahasa Indonesia kinda fits the bill, too.

It's a hard set to define. I'm tempted to go with the "second-generation  
L1 speakers" thing, but that I suspect locks out dying or dead languages  
going through a resurgence.