Bonobos' calls may be the closest thing to animal language we've seen

Great overview of more complex vocal communications. Farley Mowat makes a similar case for wolves in Never Cry Wolf: https://en.wikipedia.org/wiki/Never_Cry_Wolf

And almost certainly: cetaceans too.

Every single one of these species need the "Whale-SETI" approach applied to them.

We're probably in for quite the shock when we finally decode their communications.