system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how did we get to understand any extinct languages?

For instance hieroglyphs or other symbol based languages. How did we find out what those symbols mean, when we’ve got nothing to compare them to?
gpt
We need something like the Rosetta Stone, a stone that had the same text written in ancient Egyptian, but also in two forms of ancient Greek as well. And since we could compare both forms of Greek and see that the stone had the same text for the two forms of Greek, it was then easy to assume that it also had the same text in Egyptian as well, and we were then able to begin translating by comparing the Egyptian to the Greek. 

Without something like that though, we really can't decipher an extinct language with no modern speakers.