For some reason the docs are not on dev.openacs.org but you can
find them in packages/acs-lang/www/doc/ in the distribution. In particular there is a section in i18n-design.html "VI.B Naming of
Template Files To Encode Language and Character Set" which might have
some useful information. I am not entirely sure if this stuff was
implemented and you should be aware that Lars and his Collaboraid gang
are currently doing a major overhaul of acs-lang so even if it did
work it might not on the HEAD (then again maybe it didn't work and now it will).