Which language do you think the author and his grandmother used while talking to each other?

The specific language that the author and his grandmother would have spoken to each other is not explicitly mentioned. However, considering that the author’s family comes from the Punjab region of India, it’s likely that they communicated in Punjabi, especially since the grandmother’s education seems to have been in traditional and religious studies, rather than in English or other languages. Punjabi would have been the common tongue in their village, and it’s reasonable to assume that this would have been the language they used to communicate with each other.