If an inscription was found in a temple, for example, and it was an ancient language, how do historians translate that inscription with any degree of certainty?
Not to discourage further discussion, but you might be interested in these previous answers:
If we had never discovered the Rosetta stone, would we still have deciphered Egyptian hieroglyphs? by u/farquier
When and how were cuneiform languages (like Akkadian and Sumerian) first deciphered by modern scholars? by u/WedgeHead
When did the modern understanding of the history of Bronze Age Mesopotamia develop? by u/Bentresh