blog: bcp47: discuss overlong iso639

[srl295]

https://www.ietf.org/rfc/bcp/bcp47.html

§2.1 Syntax language

"shortest ISO 639 code”

§2.2.1 Primary Language subtag

When languages have both an ISO 639-1 two-character code and a three-
character code (assigned by ISO 639-2, ISO 639-3, or ISO 639-5), only
the ISO 639-1 two-character code is defined in the IANA registry.

§2.2.4.3.d

       UN numeric codes for countries or areas for which there is an
       associated ISO 3166-1 alpha-2 code in the registry MUST NOT
       be entered into the registry and MUST NOT be used to form
       language tags.  Note that the ISO 3166-based subtag in the
       registry MUST actually be associated with the UN M.49 code in
       question.

example eng-US is invalid, as is en-840 or en-USA