"How Much is Said in a Tweet? A Multilingual, Information-theoretic Perspective"
I think what you want is presented in a figure on page 6.
So while Japanese tweeters could express more than English tweeters in the same character count, they generally don't choose to. That's pretty interesting.
Etruscan was written in a predecessor of the Latin alphabet we still use (albeit omitting most vowels), so no, it's not going to have a huge amount of information.