"What languages would be of enough importance for, for instance, Americans, to make it obligatory education?"
My aunt forced her young children to learn conversational Spanish because she was convinced that "they're taking over." It was extremely misguided, but her children actually benefited from it, gaining both a language and some cultural understanding. How many misguided parents actually pull that off?
On a more general note, an internationalization of Americans probably won't occur until a few things happen:
1) The ridiculous taxes+fees on international plane tickets are lifted
2) It becomes en vogue to go to for vacation
3) Some famous people move to X and bring reality TV with them
or..
1) America enters a position where it is obviously no longer economically dominant BECAUSE of a lack of worldliness.
2) (1) becomes common knowledge
Until either of those scenarios occur, there's absolutely no reason for Americans to learn another language. Most Americans work and think locally. They're at least 500 miles away from another country, and they're not about to get any closer