I have answered this - Ram Sury's answer to Did Gautama Buddha preach in Pali or Magadhi Language? Indian mythology does not say anything about the Buddha's language. But it is funny that the Pāli (or Māgadhī) language was not known at all in India in BCE. read more
It should be Magadhi. Because, Pali (पालि / பாளி) was not a vernacular language spoken during the period of Shakyamuni, Siddhartha Gautama. As Magadhi Bhasha is believed to be Siddhartha's mother tongue, it is highly likely that he delivered his teachings in that language. read more
Do we know, or have a good idea, what language the historical Buddha (Siddhārtha Gautama) spoke. I've just assumed it is Pali but a moment's thought tells me this is probably wrong since the Pali canon was written hundreds of years after the Buddha died. read more