There is an interesting article in New Scientist for those in search of real challenges! The paper lists eight groups of scripts that cannot be read (or more precisely understood). The depth of the paper is however quite limited, with a mention that the pre-Roman Etruscans were “a prehistoric civilisation”, which contradicts the very meaning of pre-historic since they had an alphabet…and the first sentence of the paper namely that writing “made history possible”. This issue of unreadable languages is an interesting problem mostly when the alphabet is known, because then you can start to think of building associations with other languages of the same era and area, provided enough material is available, as is the case for Etruscan, Linear A, and Meroitic (again described by the paper in a contradictory way as hieroglyphic, while being an alphabet). When the coding structure (script) itself is unknown, this seems to set an impossible challenge, especially in the case of isolated cultures like Easter Island rongo-rongo, where native knowledge has disappeared and all existing material (26 texts) is already recorded.