Ohta-san,
as I read ISO 639 extensions suggested, they don't seem to be
progressing towards sublanguages at all.
But wasn't the suggestion unwelcomed by ISO?
Linguists say that there are 4000 or 8000 languages (neglecting dialests)
in the world, and 26^3 is merely 17576.
So, ISO 639 in the future (not necessarily the next revision) must support
language codes with 4 or more letters
or
sublanguage extension
If ISO 639 does come up with a sublanguage registry, we will just
have to change our standard to reference the new one properly, I think.
The idea that we must change our Standard just because ISO has done
something they should have done already in a way incompatible with
our way is, IMHO, quite unpleasant.
Masataka Ohta