What I mean by this is: has there ever been a country where the bulk of the population spoke two different languages as a matter of course?
This question is somewhat inspired by this video in which Simone Giertz says that "everyone in Sweden learns English from when they are eight." To my American ears, she speaks nearly flawless, unaccented English to the point where I don't, as a native speaker, think I'd even realize she was not one as well. It also brought to mind a scene in the Norwegian film Trollhunter in which some Norwegians run into some Polish plumbers. The subitled conversation is confused at first, then both parties switch to English to understand each other. Again, the Norwegians speak English with only a mild accent.
This has also been my experience professionally, working for a company that has a high Swedish representation due to past mergers. Now obviously there's some bias as to who would be working in the US, but again, often I don't even realize someone is Swedish until they start using the language. This is in contrast to many other foreign-born people I work with where, even when their English is quite good, there's still a clear accent. I've been told by a couple of Swedish coworkers that "well, everyone in Sweden knows English." (I also have some Belgian colleagues and the situation there is similar.)
So it seems that in that area of the world, there's a situation where one "native" language is used internally while another language is used to talk to people outside the country. It got me to wondering if this is a sustainable thing. If we go one hundred years in the future, will Scandinavia have become an English speaking territory, or will languages like Swedish, Norwegian, etc. persist. In other words, is what we see today in Scandinavia a transient situation on the road to English dominance, or a move to something different, with native languages and a "common tongue". So I was curious about historical precedents.
Now I know that this has not been unusual for the elites. For instance, in Roman times, most literate Romans spoke Greek, and in many periods in Europe, the elites would speak French to each other rather than their native tongue. But I'm more interested in the bulk of the population.
Have there been periods before the modern era where the bulk of the population in some region would know two languages at near fluency. For this purpose, we can define "before modern era" as "before World War II".