Linguistically, we don't know, since we don't know the Harappan/IVC language. Genetically, recent work shows the IVC peoples were closely related to the pre-Indo-European expansion farming and foraging peoples of Iran, with some mixture of peoples closely related to modern Andaman Islanders and SE Asian farmers and foragers. Their closest modern living relatives appear to be South Asian, and in particular, southern South Asians. Modern South Asian populations appear to be a mixture of IVC peoples and Indo-Europeans, with more IVC ancestry to the south, and more Indo-European ancestry to the north.

Much of this was known earlier. What recent work has contributed to this are IVC genes. The similarity between IVC genes and neighbouring populations of the time in Iran and South Asia, and lack of Anatolian genes, suggests that farming in the IVC was an independent development by local people (possibly based on the idea of farming coming from elsewhere), and was not due to settlement of the Indus Valley by farming peoples from further west.

For the new work on the genetic of IVC populations, see:

• Vagheesh M. Narasimhan et al., "The formation of human populations in South and Central Asia", Science 365, eaat7487 (2019). http://dx.doi.org/10.1126/science.aat7487

• Vasant Shinde et al., "An Ancient Harappan Genome Lacks Ancestry from Steppe Pastoralists or Iranian Farmers", Cell 179, 729-735 (2019). https://doi.org/10.1016/j.cell.2019.08.048

For the older work on the genetics of South Asian populations, see:

• Priya Moorjani et al., "Genetic Evidence for Recent Population Mixture in India", The American Journal of Human Genetics 93, 422-438 (2013). https://doi.org/10.1016/j.ajhg.2013.07.006

• David Reich et al., "Reconstructing Indian population history", Nature 461, 489-494 (2009). https://doi.org/10.1038/nature08365