Figures for a language's speaking population are even today uncertain and difficult to ascertain. ... About all we can do is to subtract the foreign-born population from the total number of inhabitants, then make a rough estimate as to what proportion of the foreign-born may be expected to have learned the language, and add that tentative figure to the native-born population.
――英文和訳演習 上級篇 伊藤和夫 駿台文庫