If you were given the job of creating a new "Rosetta Stone", which three languages would you choose, and what would you write on it?

What's wrong with Sumerian, Egyptian, and Greek?

C++, Java, and Python.
Because I know a lot of all of them.

Geek, leet, and Valley?

I guess I would go for:
English (or Dutch, German, etc)

Why? Because these three are very different from each other. From char-set to idiom and grammar.

This would increase the change that someone could translate at least one of them.

English, C and Babel (as in the fish).