Which language did the Buddha preach in?

The Buddha spoke the dialect called Magadhian, the vernacular spoken in the ancient kingdom of Magadha, which was located around modern-day Bihar. Although most refer it as Pali; the Pali language is not actually the name of a language. Pali means "language of the text" which refers to the Buddha's discourses.

You may like to read Wikipedia here:en.wikipedia.org/wiki/Pali.