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Ooh there's a cool idea, I hadn't thought of that.
Another angle is the possibility that vastly-improved directly-implanted translators - a babelfish, basically - might make the whole thing moot. You learn your first language and then have absolutely no need, ever, to learn another. Language could be more or less frozen wherever it stands at the time. That's if the technology is universally available - things get even more interesting if it was only available to the wealthy, or to citizens of wealthy nations.
Second language might still be necessary for the cognitive development effect.