So, maybe we can estimate a different way. The Ethnologue has data on the estimated number of speakers for each language within a country, along with the number of people in a country. Subtracting the number of speakers from the number of people gives, in theory, the maximum number of bilinguals in a country.
Maximum Number of Bilinguals =For example, if a country has 1 million people, and 500,000 speakers of language A and 750,000 speakers of language B, then 250,000 must be bilingual (if there are no other languages spoken). The figure below shows the ratio of speakers to people with darker areas indicating higher levels of bilingualism (data from Ethnologue, created with R):
total number of speakers for all languages – total number of people
At any rate, there appears to be no correlation with latitude (r= -0.1, t = -1.4, df = 197, p-value = 0.15) or longitude (r = -0.01, t = -0.28, df = 198, p-value = 0.8).
Ah well, back to counting people instead of numbers.
Gary Lupyan, Rick Dale (0). Linguistic Structure is Partly Determined by Social
Structure in Press