I've been reading a number of books recently dealing with how various languages were discovered, analyzed, and translated in the past. If the language itself is unknown, the key has almost always been what's referred to as a 'bilingual', some text of sufficient length written in at least one known language as well as the unknown.
The most famous example of this is of course the 1,700-lb Rosetta stone, with the same text in Greek, demotic, and Egyptian hieroglyphs. The Rosetta stone allowed Champollion (arguably) to finish deciphering the hieroglyphic language by means of comparing proper names in the Greek and attempting to find the equivalent hieroglyphs.
I started thinking about how various languages might be deciphered in a distant future when humans were long gone. It occurred to me that about the only real stone inscriptions in any quantity are those that appear on grave stones, and the length of each inscription consists almost entirely of names and numbers if not exclusively. Granted, there are also some monument inscriptions and thing of that nature.
As more and more things become electronic and internet-bound, it may appear to some future civilization that at some point we just stopped writing. But what about plastics? An increasing number of things are plastic, and it's well-known that plastics last a ridiculously long time before finally decomposing. Something as simple as Coke bottles discovered in the landfills of various nations may someday allow many human languages to be unraveled. It's an odd thought that our garbage may be the only real clue to our civilizations.