What language did King David speak?

Hebrew, as is abundantly clear from the Book of Psalms. See also:

More about King David

About the Psalms

More about the Hebrew language

If King David was a real person who ruled at the very beginning of the tenth century BCE, he might have spoken a form of proto-Hebrew, which would have been no more than a dialect of the parent Canaanite language. Hebrew, as a separate language, is believed to have developed no earlier than about 900 BCE.

* Hebrew

* David never spoke Hebrew because Hebrew did not exist until the exile into Babylon. When Jews were in Babylon they made up Hebrew. David spoke Syriac, Egyptian and another Caananite language. Syriac is the same as Aramaic, the language of Yeshua (Christ).