This idea comes from the Russian Wikipedia discussion about Compact Language Links: the territory-language data CLDR doesn't necessarily correspond to the popularity of Wikimedia projects in each country. In Russia, for example, English is not listed as one of the languages in the CLDR data, but the English Wikipedia is the second most popular edition according to the per-country reading breakdown at stats.wikimedia.org (albeit with modest 6%).
So the data from CLDR can be augmented by using the statistics about site visits, stats.wikimedia.org (or hopefully, something more modern and detailed).