Actually Warlocks could understand demonic they just couldn't speak it. When the bosses in Alcatraz would speak demonic I could always understand them as a Warlock back in TBC. It depended on if blizzard flagged the "demonic" as an actual language or just an emote/whatever. Warlocks couldn't speak it until they would /say in metamorphosis and it would come out as a demonic.
There are a lot of things that should be added back to Warlocks. Maybe not speaking Demonic but at the very least understanding what any Demon or Demon Hunter says in demonic. And tracking/sensing demons. It never really has a use but the Dreadlord Infiltrator launch event shows how silly it is that Warlocks can't.