1. What is the most linguistically diverse country in the world (by number of languages per capita)?
Vanuatu. There are 106 languages spoken in this small island state, while the whole population of the country is less than 250,000.
2. What do you call a specific language of an individual person?
3. What do you call a feature of the human language that allows us to communicate things about the language itself?
It is the metalinguistic feature.