Q: Do you believe that the Indus script represented a single language or could it have represented a multiple number of languages?
A: This is a very interesting question. I will give you the parallel of the Chinese language. The Chinese have only one script but they have many mutually unintelligible dialects. Such a situation might have existed in the far flung areas of Harappa, but I believe basically that the Harappan script was intelligible to all the people of Mohenjo-daro and Harappa. If there were differences, it would have been more diacritical than linguistic.
The very strong reason for this was the one found out by Hunter long ago. Hunter pointed out that the frequency and combination of signs at Harappa and Mohenjo-daro are the same, showing that the language used there must have been the same - subject to what I said earlier that there might have been diacritical differences. If you take English and French, both are written in a form of the Roman script but the frequency and combination of characters in the two languages is different. Therefore, all over the greater Indus Valley the language was the same and it was the same throughout the civilization. There is one major exception to this and that is very interesting, again pointed out by Hunter, and recently restated by Parpola and his colleagues with more evidence.
In some of the Indus Valley seals found in the Middle East, particularly the round seals which must have been locally manufactured, the order of the signs and their combination are totally dissimilar to what we find in Harappa and Mohenjo-daro., This may have been an attempt to use the Harappan script by the natives of the Indus Valley who went over to the Middle East for trade purposes, to adapt the Indus script to a local Middle Eastern language. But for this exception, within the Indus Valley itself, in all its areas and throughout the time, the language was the same as proved by the frequency distribution of the Indus signs.