I have a trivial reason (literally, it's trivia) to think the Anunnaki spoke Sumerian and taught it to us humans.

The Voyager space probes that were headed out of the Solar System contained golden LPs put together by Carl Sagan with various types of data. One part contained greetings in 55 languages. The very first one: Sumerian.

That should be noted as odd because the last native speaker died about 4,000 years ago. Why would Sagan include it?

In the photo section, there are only two pictures of Earth from space: one of Egypt and the other of Sumer. Why those places and not, say, the good old USA where the probes were launched?

I think Sagan was hip to the Anunnaki. In the end he chose a career as a science popularizer as a better way to get through to people. But I think he could not resist including a wink and a hat tip just in case someone out there might recognize the sights and sounds of their home away from home.