'si' documentation

it appears that whoever created the slovenian (si) documentation tree,
(probably me) didn't double-check the iso language codes -- slovenian
should be 'sl'.

any objections if i straighten this out? this will break existing
checkouts, but it seems better to fix it now before people get too
attached to the incorrect name. and before someone comes along and wants
to translate the documentation into sinhalese and finds their language
code taken.

jim