In short: By the time a person is 18, they must effectively be able to communicate and understand conversationally in 2 languages and casually use them in daily life..., if not become completely fluent...
Other than that, any language goes (whether it is a locally-known one, or a popular one worldwide),
The only thing I hope to gain from this, is to rid the world of /Monolingual Betas/
Seriously though, has this been a policy before? Because I haven't heard of such one...
I think this can especially be used for citizenship...
Edit: I don't necessarily have any other presupposed requirements besides bilingualism, though we may have certain notions of such in this main goal
Edit II: In furthering this venture, I have realized that my liberalism may slightly poisoned my lens....
And for clarification...
Minimum dual language system:
Main national language + other language (likely another related language, but foreign ones are fine)
Data here is a few years old, but in most places in the EU, more people can speak at least one additional language than not.
https://jakubmarian.com/average-number-of-languages-spoken-by-the-eu-population/
I'd wager a big part of it is due to the legacy of colonialism, where in the Anglosphere there are so many people in every direction that speak the same language that there isn't the same incentive to learn another. Whereas, for most of human history, people have needed to learn additional languages, and often stay accustomed to learning new ones.
A conjecture you could make is that for any country where over 90% of people speak the primary language, then the more populous the country, the less likely people are to be bilingual. This explains Japan, and perhaps also Russia and large parts of Latin America.
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