I am a Southerner, and I had to drop y'all when I moved to California. In contrast to TerribleT's experience, people did look at me twice. So it's non-Standard English. It's not used in most of the U.S., and certainly not used in Canada, the U.K., Ireland, Australia, or New Zealand... (although a small minority in each location might be using it, for all I know).
So that's why y'all is currently not accepted. Simply due to the mechanics of setting up what is a free program, after all.
In addition, I'd add Y'all are not sad? doesn't sound natural to my Southern ears. We'd use the contraction: Y'all aren't sad?
What it's doing is giving you the pieces of the word. So it'll show:
n't , are,
like that. And you choose the words for the sentence. So if the sentence should translate as: he doesn't eat. You'd choose : he, does, n't, eat.
If you look at it once you've chosen those, it'll read: he doesn't eat.