My copy of the original message seems to have gotten lost in the mail
storm of the last few weeks (might turn up yet), but as far as I can
deduce from your comments, Harald, it seems plausible. In principle,
it could result in IANA having to register a very large number of languages
that do not appear in ISO 639. But that would probably be ok if it
happened. And, unless folks give into the temptation to show off how
smart they are by registering languages that are not actually used in
email communications, it won't happen in practice.