I agree that Spanish and English are similar, but how would someone without knowledge of Latin be able to really tell?
Well, the letters actually look the same.
If both were provided, then translations would become easily, which would allow for quick and decisive translations for the text created by a very large portion of the world's population.
I think the Rosetta Stone only works because we have knowledge of one of the languages prior to the discovery of it.
I agree that the Cyrillic alphabet would provide more diversity, but considering Russia's population, it might not be the most important. Maybe written Hindu/Urdu?
Maybe, I just know Russian looks crazy.
Though I guess if we were to make a modern Rossetta Stone, there wouldn't be that much preventing us from including almost all of the main languages of the world.
We don't know what we need to preserve against. Putting every language may be too much of a pipe dream.