Note:
we already have a header Language: defined in an IETF standards-track
document - RFC 1327.
It defines it as (partial BNF):
..... "Language" ":" language
language = 2*ALPHA [ language-description ]
language-description = printable-string
and is indeed referring to ISO 639.
NOTE: As I learned to my pain, linguists think that ISO 639 define
approximately 10 % of the world's languages, and a lot of them wrongly.
An ISO committee is drafting a new version. Don't expect results soon -
it seems that linguists are as quarrelsome as networkers! - so we can
indeed regard 639 as a "stable spec".
Harald Tveit Alvestrand