Considering the fact that the Harappan script may have been proto-Brahmi, the underlying language to be expected should be Sanskrit, or proto-Sanskrit, or derivatives of Sanskrit. Many of the rules of evolution that apply to scripts are equivalently true for languages too. Like scripts, languages too render themselves to similar evolutionary inspections, as they too carry imprints of their journey down the ages.
Subhajit Ganguly, Call Of The Lost Ages: A Study Of The Indus Valley Script
brahmi, deciphering, harappa, history, india, indus-valley, script