Perhaps you are familiar with the book Carry On, Mr. Bowditch by Jean Lee Latham.
If you need a refresher, Carry On, Mr. Bowditch is the story of an early American merchant who spends quite a bit of his time at sea. As Bowditch sails around the world selling his wares, he teaches himself various languages. He begins with Latin. After Latin, he teaches himself Portuguese, Spanish, and French.
How does he do it?