I have recently been studying the Indus Valley Civilization, its cities (Harappa and Mohenjodaro), and what we know about its culture. The thing that strikes me is how little we know about their language and how little the scholars discuss it (at least the ones I'm reading). I was wondering what theories are being tossed around in the historian community. Was it pictographic or did it have grammar? How decipherable is it? Will we ever find something similar to the Rosetta Stone (there is evidence of trade tied to Mesopotamia) to decipher it?
I hope that someone here can provide an answer. But, if not, you might try x-posting this question to /r/linguistics.
I believe the excavation at Harappa/mohenjodaro unearthed pictograpic "writing" on steatite seals, but it was/is unclear whether or not there were enough characters/alphabet for the written language to fully represent the spoken language. The examples ive seen had the writing as a smaller part on the seal, with most of the space used for an image of an animal or person. In fact there was suggestion what was found was not writing but had some other purpose.
See if you can pick a copy of Mortimer Wheeler's book The Indus civilization. He has a brief section on the script, notably saying: only 396 unique characters found, has a boustrophedon pattern (you follow it around like a snake instead of line by line), and that the longest fragments of continuous signs is about 17 and most only around 12. He speculates only perhaps it was used for proper names. Keep in mind this is quite an old source so new finds and new research has probably been occurred since Wheeler.