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)
It would be so handy to communicate with someone from the other side of a window or something too
I expect everyone would develop a habit of signing some things while talking, to emphasize a point or provide context.