What I mean by this is, has there every 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 Swedish 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 think I'd 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 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, 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 "modern era" as "before World War II".

Gilbratar

English is the official language in this British Overseas Territory, but the populace is also highly fluent in Spanish due to physical proximity and social interactions with its larger neighbour. In fact, bilingualism is so deeply entrenched, the population of Gilbratar routinely engage in code-switching in everyday speech.

That is, they switch between Englishi and Spanish while speaking. The result is Llanito, which Wikipedia describes as "speakers appear to switch languages in mid-sentence".

Yanito (or Llanito) is the name popularly given to the native of Gibraltar as well as the local verncular he/she speaks . . . Dating from the early or mid-ninetheenth century, it came to be used to refer to the poeple of Gibraltar who, at that time, were predominantly Genoese.

_{Levey, David. Language Xhange and Variation in Gibraltar. John Benjamins Publishing, 2008.}

Given that Llanito emerged in the 19th century, we can surmise that Gibraltar must have already had a fully bilingual population by then. That is well before the deadline set by the question.

Luxembourg

Bilingualism has been enshrined in Luxembourg's basic laws since the Grand Duchy's first constitution, drafted in 1848. In fact, bilingual instruction were mandated shortly after the modern Luxembourg achieved de facto independence in 1839:

In accordance with the Education Act of 1843, basic literacy was taught via standard German, and standard French was taught as an additional language in primary school.

_{Kaplan, Robert B., Richard B. Baldauf Jr, and Nkonko Kamwangamalu, eds. Language Planning in Europe: Cyprus, Iceland and Luxembourg. Routledge, 2016.}

The net result of the committment to bilingual (and later, trilingual) instruction is to create a trilingual populace:

Trausch . . . concludes that . . . 'from [the time of] the partition of 1839, the Luxembourgers became accustomed to using three languages in the course of their everyday life: their "native" Luxembourgish, German and French.'

Like Gilbratar, this everyday multilingualism become entrenched well before the Second World War.

Depending on the desired timeframe, that is: how long such a state should have been in existence, or region being in such a satus, this might get to a very long list.

From Hellenistic times onward, we can assume that in for Egypt the people would have understood Coptic and Greek, and later added Latin to the mix. The Rosetta stone series of mulitlingual stela come to mind easily.

Equally in Palestine, a mix of languages co-existed with people conversant in Hebrew and Aramaic, then Greek and Latin as well.

If you look at a European map of languages before the World War you'll find Gaelic speakers using English, Alsatians speaking French and German, Belgians speaking Flemish and French, Schleswigers speaking Danish and German and Sorbs speaking Sorbish and German. Further East you'll get Polish/German, Kashubian/German/Polish, Hungarian/German etc. And this is not only considering societal elites but ordinary village people as well.

The oldest German University is in Prague. While the town spoke nominally Czech, Pragerdeutsch was considered the purest dialect of all German variants. Before the war Low-German (linguistically arguably an altogether different language from Standard German, and before the war most people there were raised Low German but learned their first foreign language in school: German.

As a general trend: Whenever a lingua franca was being established, or foreign-language settlers come along you can bet that most of them will learn enough of two languages to trade, argue with and insult their neighbours. Whenever there is a language border, all along the border region most people will learn at least some of the other languages.

Although some are too arrogant or stupid to do so. Some also take pride in not knowing or pretending to not know another language. Go to France and experience a bit of that now. Go to multilingual Switzerland's nominally German-speaking parts, and realise that most people can speak excellent German indeed, but most refuse to do so on an everyday basis, opting instead for Schwiizertüütsch, which is so different from Standard German that to follow a conversation becomes very difficult.

The most interesting thing comparing Swiss German with Low German is that the temporal development is exactly opposed. After 1950s this kind of bilingualism declined for Low German as speakers were socially discriminated against for a few decades, while in Switzerland the Standard German lost considerable prestige compared to the local variant. Not in the least because Low German is now linguistically often seen as a distinct language and raising awareness for local patriotism, the trend towards extinction is tried to be countered both officially and on private initiative. It further seems that this conservation of variants and minority languages is now part of European Union policy.

If you are curious about very dynamic bilingualism within one conversation, then it is perhaps something like Code-switching in Hong Kong?

I heard Pennsylvania Dutch speakers have a pretty good command of English too…

Of course: Vatican_City. Official languages: Latin, Italian

Swiss Confederation has 4 official languages.