1.5 Normative References reads in part:

• • • • • 
          RFC3066] H. Alvestrand, Tags for the Identification of Languages, http://www.ietf.org/rfc/rfc3066.txt, IETF RFC 3066, January 2001.

As reported by http://www.rfc-editor.org/in-notes/rfc-ref.txt, the correct citation reads:

[RFC3066] Alvestrand, H., "Tags for the Identification of Languages", RFC 3066, DOI 10.17487/RFC3066, January 2001, <http://www.rfc-editor.org/info/rfc3066>.

However, http://www.rfc-editor.org/in-notes/rfc-ref.txt, also reports that RFC 3066 was superseded by RFC 4646 and RFC 4647.

Following those references, you will find that RFC 4646 has been superseded by RFC5646, for which the correct reference is:

• • • • • [RFC5646] Phillips, A., Ed., and M. Davis, Ed., "Tags for Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009, <http://www.rfc-editor.org/info/rfc5646>.

The correct reference for RFC4647 (which is still valid) is:

• • • • • [RFC4647] Phillips, A. and M. Davis, "Matching of Language Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September 2006, <http://www.rfc-editor.org/info/rfc4647>.

I don't know how you were using RFC3066 so can't comment on the impact of the superseding of that RFC by those that follow and their split into separate parts.