this isn't an issue if the language codes are used. ISO 639 is also not
very extensive...which is perhaps why it allows use of UDC language numbers
also...my guess is that the librarians care a lot more about being able to
classify documents according to their obscure dialects than the ISO people
did.
As to brevity: ISO 639 seems to have been designed to attach a language
symbol to a description of a document -- thus XYZZY (Fr) denotes a French
version of document XYZZY. The people who designed this valued brevity --
they wanted a compact representation for experts, not a user-friendly one
for ordinary humans.
Amusing aside viz theorizing about why IS 639 is the way it is: It
comes out of the library/"information sciences" community. Both the
desire for brief codes and the list of languages to include where what
they thought they wanted, for better or worse.
On the other hand, the Dewey system has to accomodate books *about*
languages, while, as you have noted, one of the original design goals
for 639 was to denote the language in which a book or article was
written. The number of languages in which things are published is far
less than the number of languages that are spoken or discussed, which be
another way to account for the discrepancy.
john