MediaWiki<->TWN relationship is an interesting topic.
We are basically outsourcing translations to translatewiki. We take any
translations/languages they provide.
So your problem could be reformulated to "How can I add a new language
to translatewiki?"
Translatewiki is a tool quite good for translating languages but I don't
know where are such topics dealt with. In some kind of Village Pump, I
suppose. Does it have a *community*? Or is it just people translating a
few messages and leaving?
I recently had the opposite problem (wanted to remove a language
translation) but didn't find out the proper way, either.
Some translators asked about one of my tools translation and I didn't
notice until after I got an email for a different thread on that page (I
didn't know I should be watching it!).
Roan notes that not every MediaWiki developer has translator rights.
The full steps for adding a message/project seem to be known just by a
few folks.
So I think there are some communication problems regarding translatewiki.
They may just need a few explanation pages, or need a more work, but
it's sane to discuss it.