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.