One quick final note on this topic. In rereading part of David Crystal's
_Language Revolution_ this afternoon (for another project), I noted that he
mentions 40% of the world's languages "have never been written down" (p. 94,
with reference to an earlier discussion I didn't find).
These "unwritten" languages are presumably mostly languages with few
speakers in
poor countries, which would mean that the languages the vast majority
of people
use have some sort of writing system. Which is not to ignore the fate of the
others, of course, but this would mean that in principle, lack of writing is
not a problem for pluriliteracy for most people in multilingual
contexts (there
are other hurdles, but lack of writing is probably not among them in most
cases).