Cabal, Hive, and Fallen are able to learn human languages, and usually important ones do, like Kells or Hive gods. We don't tend to learn their languages, although some people do such as Eris and Toland learning Hive Runes. Our Ghost can translate very language for us. Throughout the franchise, he's translate Hive, Fallen and maybe Cabal, as well as rasputin.