Yes, the major up and coming countries in the technological world are China and Japan. Although the United States is becoming more and more inhabited by first or second generations of Hispanics, it would be in our best interested to learn the language of countries that will soon overlap ours in education and technology.
Learning Spanish is the most important 2nd language to learn in America. There are many Hispanic people and a lot of them are either bilingual or don't speak English at all. This is very important in the state that I live in. Half of the people here are Hispanic. So it is very important.
I feel that there is not a second language more important to learn in America than Spanish. Spanish is the second most popular language in the United States and it continues to grow. The hispanic demographic is continuing to grow and with looser boarder policies this number will only increase in the future.
It depends on what you want to do with it, but generally, Spanish would be the most helpful second language to learn. Spanish is the most spoken language in the United States other than English. Learning Spanish would help a person get a job in the United States. Knowing Danish, probably not.
It's not uncommon knowledge that the Spanish-speaking population of the United States is very large. Not only that, if you live in states like California, you know that Latin culture is prevalent. Being able to speak Spanish not only helps in communicating with a large portion of our country but also helps us to better integrate with the culture around us. Not to say learning other languages is not worth some ones time, I simply feel you would get the most benefit out of knowing the Spanish language.