What language do angels & demons speak???

You can always just make up a language. You don’t actually to show the made up language, you can just identify. That is how its was done in the Death Gate Cycle. Elves have their own language, as do the dwarfs, and the 2 other races the authors created. They has a couple phrases in Elvish and Dwarf, but never show an entire conversation. If one of the characters can’t understand that language, the book just mentioned that, and will show the conversation in just regular English.

Maybe Hebrew? If you are talking about angels and demons in the Bible then it would be probably be hebrew. My guest is that it was the first language in the Bible, which Adam and Eve spoke and that it was a language able to capture the essence of the animals that Adam was naming. Then came the tower of Babel and suddenly there were these new languages. I seem to recall reading an article(not in the Bible)that these other languages were deemed less pure and that Hebrew was some sort of a special language. I think it is Hebrew anyway.

If I am wrong and you still want to ground it on the Bible then let us just say whatever language Adam used.

Or you can consult a linguists and semiotician maybe. And I can recommend you some literature: Serendipidity by Umberto Eco The Search for the Perfect Language by Umberto Eco

Goodluck!

Demonic , don’t know bout angels