[cedilla]

Is there any reason to assume any shared structure for unrelated languages though? Written language is just an encoding for information.

There is a good candidate for a test. Someone will probably already work on it. Minoan as written in Linear A has only survived in a few thousand tokens and despite thousands of man years of effort, natural intelligence has made virtually no progress in understanding it. That's still easy mode, since we know that the Minoans were in contact with speakers of indo-european and Semitic languages, and writers of hieroglyphics and phonetician script, so their written Language was probably influenced by that.

[KRAKRISMOTT]

There is no reason to, as stated before, it is however a necessary assumption. It is also possible that the assumption is entirely wrong, and the LLM generates a plausible explanation to their language that we cannot falsify. If the shared structure hypothesis is incorrect, then it is no different from dealing with an alien language. (Note we can also feed in related information like where it was found, what the nearby pottery shards at the excavation site are etc. I am lumping all of these under the "shared structure" banner of the LLM's model of humanity/human languages)