Marnix’s plea for early multilingual education is contained in a booklet written in 1583 for a brother of William of Orange and first published posthumously in 1615. It may well be the first recorded defence of immersion schooling:
One must approve the practice of those who endeavour to teach their children the knowledge of two languages that differ from each other by their pronunciation and vocabulary. By doing so, one will accustom their tongues, then still flexible, to two different methods of pronunciation and one will make them more apt to express later the dialects of any foreign language. […] Nor should the parents forget, if at all possible, to teach their children to speak two different languages from an early age in order to prevent their organs, once used to one accent, from sticking to it. This is why I would like Germans to learn in early childhood, at the same time as the German language, a French, Italian or Latin dialect, so as to prevent them from introducing germanicisms into other languages, as they would if used to German only; I would like that the French should be taught, together with the French language, the Belgian or Germanic or English or else Italian language, and that soon, when they will be in their sixth or seventh year, they should get used to pronouncing correctly the Latin language, which they will be taught, not by a Frenchman, but by someone else unaffected by gallicisms. When they will be older, I want them to go abroad and would like them to learn, as far as possible, the languages of foreigners, especially of those who interact with their compatriots.
(Ratio Instituendae Juventutis, first published in 1615, Latin edition (J. Catrysse ed.), Caracas: Universidad Central de Venezuela, 1959, pp. 29 and 109; French translation: Traité d’éducation de la jeunesse, Bruxelles : Editions ARSCIA, 1959, pp. 35 and 91).