Short answers, pulled from the story.
Multilingualism is the use of more than one language, either by an individual speaker or by a group of speakers. When the languages are just two, it is usually called bilingualism. A person who speaks several languages is also called a polyglot.
It is believed that multilingual speakers outnumber monolingual speakers in the world's population. More than half of all Europeans claim to speak at least one language other than their mother tongue, though many read and write in only one.
A study in Switzerland found that multilingualism is positively correlated with an individual's salary, firm productivity, and gross domestic product, with the authors stating that Switzerland's GDP is augmented by 10 percent by multilingualism. In the United States, a study by O. Agirdag found that bilingual people earned around 3,000 dollars more per year than monolinguals.
A study in 2012 showed that using a foreign language reduces decision-making biases, with the framing effect disappearing when choices were presented in a second language. A 2014 study found that people using a foreign language were more likely to make utilitarian choices in moral dilemmas such as the trolley problem, which researchers attributed to the reduced emotional impact of a non-native language.
There is no evidence for a bilingual advantage in executive function, and current meta-analyses find no effect, with the earlier idea persisting in part due to publication bias. Bilingual and multilingual individuals are, however, shown to have superior auditory processing abilities compared to monolingual individuals.
A hyperpolyglot is someone who knows more languages than the five or six that most polyglots reach, with Michael Erard suggesting eleven or more and Usman W. Chohan suggesting six to eight. Giuseppe Caspar Mezzofanti was an Italian priest reputed to have spoken anywhere from 30 to 72 languages.
Widespread multilingualism is a form of language contact that was common in early times, when small communities needed two or more languages for trade, and it persists in places of high linguistic diversity such as Sub-Saharan Africa and India. Linguist Ekkehard Wolff estimates that 50 percent of the population of Africa is multilingual.