We *do* have stuff to compare them to; we find things that we know are the same text written in two (or more) languages, then we compare. The most famous example is probably the Rosetta stone, a tablet with the same text written in three languages (Egyptian hieroglyphs, Egyptian demotic, and ancient Greek).
Just for day-to-day commerce among various peoples, translations have been a normal thing for thousands of years. Finding them is tricky, not that much survives, but enough exist that we can decipher most extinct languages to some level.
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