Are the geographical stations of the Buddha’s wanderings over 45 years not known? Or why do you say that it is not certain whether the Buddha lived in India?
Also, the Magadhi Empire was in north-east India, so quite far away from Sri Lanka. What was the language in Sri Lanka at the time of the Buddha?
What is clear is that Pali or Magadhi (Prakrit) were spoken languages as opposed to Sanskrit, which had no native speakers but was a religious language. Therefore, the Buddha certainly did not want his teachings to be translated into Sanskrit. Although Wiki says about Pali:
…However, modern scholarship has regarded Pali as a mix of several Prakrit languages from around the 3rd century BCE, combined and partially Sanskritized. There is no attested dialect of Middle Indo-Aryan with all the features of Pali.