Where is Content-language defined these days?

As far as I can tell, the most recent document defining the 
Content-language header is RFC1766.  But, according to the RFC index:
1766 Tags for the Identification of Languages. H. Alvestrand. March
     1995. (Format: TXT=16966 bytes) (Obsoleted by RFC3066) (Status:
     PROPOSED STANDARD)

But RFC3066 says nothing about Content-language, other than it being moved 
to another document.  I have failed to discover what other document.
#g


------------
Graham Klyne
(GK(_at_)ACM(_dot_)ORG)