At least the Egyptians left us the Rosetta Stone,
which gave us a
chance at breaking the format.
Only because we hadn't lost one of its other languages. See the
Rosetta Stone in the Codex Seraphinianus.
But see also The Decipherment Of Linear B; we don't _always_ need a
Rosetta Stone to suss out a lost language - while admittedly that case
was a known language in an unfamiliar writing system, it was mostly
deciphered before this was realized.
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