Please provide me with some evidence that American school students are
encouraged to learn a second languauge at school, and become
bilingual. Thanks.

Frankly, I think it will be hard to prove your point. I wish more
American students were proficient in a second language, but I don't
see much in the way of official encouragement.

Here you'll find high school graduation requirements for the 50
states. Notice the frequency of the phrase "No explicit mention of
foreign language":

http://www.ecs.org/clearinghouse/39/22/3922.htm

I don't really see a reason why American students should be encouraged
to learn a second language or become bilingual unless they have a self
need to do such.