Generally the Harappan seals are considered to be a form of writing; there’s a couple of computer science geeks who claim they’ve fed in all the seals and their program has determined that it *doesn’t* hide a language. Most regard the language expressed by the script as being agglutinative, which probably isn’t surprising, since a great big chunk of Asia (and some outliers, like Turkey) speaks agglutinative languages. The largest agglutinative population in modern India (I believe) is Dravidian, but I don’t believe anyone has linked spoken Dravidian with the seals. I’d be surprised if a bilingual is never found, and I’d also be surprised if Harappan doesn’t turn out to be a language isolate, more or less.