Other arguments to consider:
* If I speak only one language, I assume everyone else in the world I may have contact or do business with to learn more than one, and to learn English. Europeans call this cultural arrogance. Learning more than one language makes us better world citizens.
* America is already a multilingual place. 17 million households speak Spanish as the primary language in the home. Spanish is my second language (and I am not close to fluent yet) and I use it in my work and the valley in which I live all the time.
* Learning a second language is difficult, especially if you are older. But learning a third language after that is easier, and a fourth even easier. Learning a second language makes you much more likely to learn a third or fourth.
* Learning a foreign language helps you understand the roots of language, words, grammar, etc. and, depending on the language, can also help you to better understand your native tongue, as there are often many similarities in language families.