These days, native speakers of English use that language in a wide variety of settings. Indeed, with each passing year, we are more and more likely to employ our mother tongue in all activities in which speech, reading, or writing plays a role, and to do so exclusively.
Things were not always so.
Consider, if you will, the case of Eben Swift (1854-1938), an officer of the US Cavalry.1 Commissioned in 1876, Swift spent most of his fifty-two year career in isolated posts along America’s western frontier, often in places where Spanish served as the lingua franca. In his leisure hours, he translated, from French into English, books about tactics, and, in particular, collections of tactical exercises.
Swift paid little attention to spoken French. (During the First World War, while many of his contemporaries served in France, he commanded the small American contingent sent to Italy.) Likewise, while he could read enough Spanish to consult statistical handbooks, most of the sources he used to compose his study of the military geography of Chili were written in English or French.2
Like so many people in the past, and, indeed, so many present-day people living outside of the English-speaking world, Eben Swift used each of his languages particular purposes. Because of this, he avoided the trap of thinking that the benefits of learning a foreign language can only be found on the far side of fully-fledged fluency.
For an appreciation of the work of Eben Swift, and its effect on the development of American military culture, see Bruce I. Gudmundsson “The Operational Cultures of American Ground Forces,” in Mikael Weissman and Niklas Nilsson, editors, Advanced Land Combat: Operations and Tactics, (Oxford: Oxford University Press, 2023), pages 234-237
Eben Swift “The Military Geography of Chili” in Arthur Lockwood Wagner et al.. Military Geography: Lectures In the Department of Military Art (Fort Leavenworth: United States Infantry and Calvary School, 1895), page 66