I’m not entirely sure about Mycenaean greek but there are old languages that get deciphered because text can be found with different languages some of which we understand. For example the Rosetta stone was written in 3 languages or writing systems:
1. hieroglyphics, 2. demotic script (a cursive form of Egyptian hieroglyphics), and 3. Greek alphabet. We know the Greek alphabet, so we could decipher the hieroglyphics. This was the key to understanding Egyptian hieroglyphs.
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