It is likely that various kinds of "proto-writing" have been independently invented in a lot of places, but very few of them have evolved into writing systems.

"Proto-writing" is just a set of graphic symbols that are used to designate various things. Such a set of symbols can be used e.g. to write an inventory, to tag things to show ownership or purpose, to show on a map what can be found in certain places, and so on.

"Proto-writing" cannot be used to write human speech. All systems of "proto-writing" that have evolved into writing systems have done that by reinterpreting a part of the graphic symbols, or sometimes even all of them, to no longer be the names of some things, but to have a phonetic meaning, i.e. to represent some sounds of human speech (syllables in almost all cases), allowing thus the writing of the more abstract components of the speech, like various grammatical markers.

Therefore for a system of "proto-writing", it does not make sense to ask which is the language that has been written with it, because there exists no such language.

The only kind of information that can be known about a system of proto-writing is which is the thing denoted by each symbol. Even when the meanings of all symbols are known, that does not offer any information about the language used by those who have invented and used that system of proto-writing.

For now, there is no evidence that the Indus script was a writing system, because only very short strings of symbols have been preserved. It could have been a writing system, because by that time other writing systems already existed not far away, which could have inspired them, or it could have been just a proto-writing system, which would give no clue about the language of its users.