From Fernand Braudel’s The Perspective of the World, v. 3, p. 103: “In Bergen, young men ‘serving an apprenticeship’ might stay for ten years, learning the local languages and trading practices….”
This is how persons needing a practical knowledge of a contemporary language learned them (note plural in the quote). No classrooms, no textbooks. Only the Classical and biblical languages were “learned” via books and teachers, and those seldom led to fluency. See Waquet, Latin: The Empire of a Sign.
The retort from legacy teachers is always: “Well, we can’t send our children out like that so we need to teach them in school.” But it doesn’t happen, any more than medieval students learned Latin or Greek as useable languages after Latin stopped being used colloquially in the monasteries.