There are a few facets to this question:
- What sign language is shown in the Eternals film (is it actually ASL, BSL, etc) ?
- If it is an actual real-world sign language, are we supposed to infer that the real-world development of the sign language was influenced by the Eternals? Or are we supposed to imagine it is not the actual language they were signing but just a translation for the benefit of the audience, the same way we clearly are not meant to believe they were speaking English in ancient Mesopotamia?
- If it is not a real-world sign language, then who (out of universe) developed the signs and what was it based on?