Wasn’t it the opposite problem in Champollion’s time? All the guys who failed at breaking Rosetta were studying Arabic and Hebrew; Champollion was the only guy who bothered to learn Coptic (surprise surprise, it’s basically the same language).
Yup, that helped with figuring out pronounciation, and working the other way, with decipherment.
Champollion followed in the footsteps of some others who used the symbols in cartouches (the pharaohs' names) on both the Rosetta Stone and various monuments to worm their way in. Hieroglyphic writing was lost for more than 1500 years. Anyway, I think it's easy to forget what spectacular achievements were made in the field by people who never had digital technology (computers, cameras, email, cell phones) -- they cracked hieroglyphics and demotic, cuneiform, and a host of lost languages (including Old Persian) recorded in cuneiform, one of which only exists as a single sample on a tablet from the Amarna archive. And of course, I always like to give props to Emil Forrer...