Writing Systems of the World 10, Exercise 1: the Rosetta Stone

It helped because it carries one decree in three versions (hieroglyphs,
Demotic, Greek), and scholars could already read Greek, so the Greek text gave
a known starting point (the first translations appeared in 1803).

It was not enough on its own because, as the Decipherment article says,
"multilingual text may be insufficient for a decipherment": translation is not
reversible, and matching the Greek to the hieroglyphs does not tell you what
each hieroglyph means. The work went on in steps: Thomas Young identified
phonetic signs spelling the name Ptolemaios (1814 to 1819) and showed Demotic
was partly phonetic, and Champollion built a phonetic hieroglyphic alphabet
and announced it in September 1822. The Decipherment article calls
decipherments the result of incremental progress by a broader community.