Learning a second language, either by immersing yourself into the culture that speaks it or by being formally taught by a fluent speaker, is an old practice in history. But the scientific study of second language acquisition is a much more recent development (as late as the 20th century).
So, has the teaching of second languages been affected by these studies? What techniques and theories foreign language teachers now have available that they didn't have in the past?
In order to keep this question focused, I'm referring specifically to teaching small groups of middle class literate adults who are native speakers of an Indo-European language and who voluntarily decide to learn another IE language, but belonging to another subfamily. Assume they are learning that second language in their native country, where it is not spoken (e.g. Spanish people learning German in Spain).