Suppose you wanted to learn, say, French. I've occasionally wondered if you could do it this way:

1. Pretend that French is a dead language from an ancient culture. Obtain a bunch of French books and study them, to try to notice patterns and infer rules about the language.

2. Do the same with some other language, such as Spanish.

3. Use the manuals from your TV, microwave, vacuum cleaner, and so on that have the instructions printed in English, French, and Spanish to relate the two "dead" languages to a language you understand (English). The idea is that these manuals are your Rosetta Stone.