This will never happen because not being able to communicate cross faction is hard baked into the game the same way that your 16 slot original bag can't be replaced.
Pandaren could communicate in beta because Huojin Pandaren and Tushui Pandaren were identical. They still are, but it's mechanically different for gameplay reasons. Also in Vanilla Alpha, Forsaken spoke Common-- could easily be reverted. Also also, during the Wrath of the Lich King launch event, Horde and Alliance could chat freely as zombies.
So yeah, it's been historically not only possible, but trivially easy to communicate cross faction and one would only have to re-implement one of those features and change the name to some language everyone can speak.