From The Economist:
Daily chart: Speaking in tongues: Language diversity around the worldBriefly explains an interesting quantification of a country's linguistic diversity, based on the probability of two people chosen at random from that country having the same native language. (If two people chosen at random had 100% chance of having the same native language (i.e., everyone has the same native language), the score would be 0 (North Korea apparently fits here); if two people chosen at random had no chance of having the same language (i.e. no two people share the same native language), the score would be 1 (not actually possible in practice, but Papua New Guinea comes rather close).)
It doesn't explain the scoring in enough detail for me to tell it's as simple as that a score of 0.9 means two people chosen at random would have a 90% chance of having different languages, or if there's a bit more involved in creating the score.
It's also not clear to me if it accounts for people having multiple native languages