Similar to this question (What are the most common languages among people who DON'T speak English?), is there any data on individuals who do not speak a language (or none of multiple languages), or data on how many people speak which combination of languages, that could be used to answer the question?
The idea is to be able to determine, for any person, the language or languages that would enable him to speak to the most new persons. Eg, if someone knows Spanish, it would not make sense (from the standpoint of this question) to learn another language from Spain, even if there is a large absolute number of ppl, because they would likely know Spanish, which could be used for communication. Someone who knows Cantonese might not want to learn Mandarin because of all the Mandarin-speaking ppl who know Cantonese. Someone who knows Hindi and English might not choose Mandarin next (if there are a lot of Mandarin speakers who also know English), etc.
Does such a data set exist? It could also be in a positive form, ie would show how many people speak each combination of languages, which could be used to subtract out languages.
Edit, 9/7/19: There is at least some data on English speakers. https://www.telegraph.co.uk/travel/maps-and-graphics/mapped-english-speaking-countries/ https://en.wikipedia.org/wiki/List_of_countries_by_English-speaking_population