Void elves sadly don’t have the option to speak Shath’Yar, so they can’t naturally understand it. Shadow priests famously speak Shath’Yar while under the effects of Void Form, so that is the only window where they can understand it. Just as Warlocks could (and DHs currently) speak and understand Demonic when in Metamorphosis.
There’s a little language filter that gets flipped on when you take these forms, and when you have this buff on your character it is also flipped. Unfortunately you can’t speak it, but that would be very cool if it gave you the language.