I would surmise that a critical-historical investigation of the linguistic practices of our species demonstrates that languages are no more fixed than than anything else.
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It depends. For a very long time, roughly 1200-1600, it was thought that Latin was a superior language because it did not change. It did not change because Augustan Latin became the lingua franca of the educated and the clergy. The vernacular went right on changing underneath as did the vulgate. So, how fixed language is depends on who speaks it and for what purpose.
Joanna