Hi there!
With beta 7 or 8 (don't remember) I had the interlingua since my locale is fi and such files didn't exist on it. I did no tweaks to get it to work exept turning the voice on. If you have beta 9, it's possible that there is something wrong with it. From Mardys blog I read that quite alot has changed under the hood. As I said earlier I haven't yet tested it my self. Stefan.