In order to compete globally (which we must to improve the economy ever) we need people who are multi-lingual or we will fail miserably.
So far they've adapted to us because we're the biggest consumers by far, but that may change eventually. Maybe if America's economy was based upon tourism we'd need to learn the language of the tourists.
I don't think 2-years, or even 4-years, of foreign language classes in high school is going to make enough of them competent enough to make it worth it. Most people who have taken 2 years of Spanish classes years ago do not speak Spanish well enough for it to be useful. You'd want to start with elementary age kids, and anybody who doesn't use it is going to lose it. Immersion in a culture that primarily speaks that language seems to be necessary.
I'm pretty neutral about it though. I'd still encourage people to take a foreign language since it has benefits beyond communicating with people of that language, but I'm not sure about making it mandatory. I look at it kind of like programming. Taking one C-basic class for my non-computing degrees did not make me a competent programmer by any stretch of the imagination,
but it did improve my ability to solve problems and think.