Today learning a language is not easy, but the resources are at our fingertips and thus much easier than before, how did people in for example medieval France learn languages?
More can be said if anyone has anything to add, but in the meantime, we do have a section of the FAQ dedicated to multilingualism, including how people learned languages. I'm not seeing any older answers about France particularly, but there are some responses about other medieval states.
That section can always be expanded, if anyone has insight on languages or eras it doesn't cover and they wish to share it.