It depends on why you're learning a language.
Leisure
French - It's no secret that in America, French is considered the premier romance language. Having access to a French vocabulary could help you to woo that person you've been eying from the bushes(I kid, I kid).
German - German to the general population is pretty much the anti-thesis to French, a more burly/rough language. Overall there isn't a hugely prevalent German speaking German-American demographic, nor a lot of jobs that will open up, so it's a leisure language for Americans.
Academia
Latin - If you're going into history, most of the European languages have some root in Latin, and many Latin phrases are becoming an increasingly sophisticated form of expression since they have gone more and more out of use.
Chinese - An understanding of Chinese opens up the historical records of the other side of the world. Japanese can also be much more easily learned for one with an understanding of Chinese, since the Japanese 'kanji' is Chinese characters, and the 'hiragana' shorthand chinese characters, it is the spoken version that differs.
Job
Chinese / Japanese - A lot of American industry is moving to China or is already there, and a lot of it also exists in Japan. If you're looking to enter the field of business, the next best language to English will probably be Chinese for the future, followed by Japanese.
Arabic - It's no secret that we don't exactly have an abundance of people here in America who know about the people we're fighting, much less can understand them. Middle East relations will be tense for many years to come, so the need for interpreters in the government and in private functions will remain high.
Spanish - By and large the largest demographic of non-native English speakers on the West Coast of the US is Hispanic people. It could help you get a job managing places where workers speak Spanish, and may also have its casual conversation usages.
Other - Dutch, Italian, and Russian are probably the big three others you could learn for various applications. The next tier would be Scandinavian languages (Swedish, Finnish, etc.), Balkan languages (Romanian, Greek, etc.), or other East Asian languages (Korean, Vietnamese, etc.). The last tier of Afrikaan and other African languages, or other less widely used ones, won't serve you particularly well unless you have an eclectic job path.
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