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)
I think everyone should learn esperanto or some made up languange as a second language so that you can communicate with anyone in the world no matter what country they come from.
Idk ... in my opinion, naturally-developed languages are the basis to go, but alr then...
Maybe 100 years from now we'll have the global coordination needed to make stuff like that a reality.
Until then, I think it's a lot more useful to have "regional languages". Between Arabic, Persian, Turkish, Mandarin, Hindi, and Malay, over 3/4 of Asia speaks one of these, and roughly half of Asia speaks one of them natively. Add Japanese, Vietnamese, and Thai, and you reach perhaps 90% L1 or L2. Interestingly enough, only 2 of these 9 languages share a language family.
4 languages make up >95% of the L1s in the Western Hemisphere, and these carry over to Africa to make up probably at least 60% of people at L2.
Perhaps a controversial position would be that groups of endangered or smaller languages could maintain a better guaranteed existence by coalescing around one of them as a lingua franca, and possibly having better prospects of outsiders learning that language too.