Is Natural Language Turing Complete?

Hi all!

Does anybody know if exists a formal proof of the subject?

Reading about NLP in various books I've never found a "common" grammar for 
english or any other natural language so: does it really exist? And... what 
about disambiguation? That's to say IMHO for every natural sentence there 
are always more parsetrees....

So there are people who speaks about NLs in context-sensitive language (so 
an LBA should be sufficient) or NLs in RE... but I've never found a formal 
Proof....

IMHO I can always find a phrase wich an NLP system is unable to parse....

Thank you,

Silvio