That seems a little circular? If all it needs is a community, and you 'bring it back to life' in a new community, then the language has all it needs to be 'natural'?
It's not a community but the community. When a language is dead, how do you learn it to native proficiency without native speakers to help you? What does it mean to a child to learn it as a mother tongue when it's not the mother tongue of the parents or anyone else alive?
