A 2002 study "The returns to speaking a second language" by Saiz and Zoido http://www.phil.frb.org/research-and-data/publications/working-papers/2002/wp02-16.pdf found that learning German boosts earnings more than learning French or Spanish, because the supply of people who know German is small relative to the demand for them.

http://www.economist.com/blogs/prospero/2014/03/language-study
Johnson: What is a foreign language worth?
The Economist
Mar 11th 2014

Quote
Second, Albert Saiz, the MIT economist who calculated the 2% premium, found quite different premiums for different languages: just 1.5% for Spanish, 2.3% for French and 3.8% for German. This translates into big differences in the language account: your Spanish is worth $51,000, but French, $77,000, and German, $128,000. Humans are famously bad at weighting the future against the present, but if you dangled even a post-dated $128,000 cheque in front of the average 14-year-old, Goethe and Schiller would be hotter than Facebook.

Why do the languages offer such different returns? It has nothing to do with the inherent qualities of Spanish, of course. The obvious answer is the interplay of supply and demand. This chart reckons that Spanish-speakers account for a bit more of world GDP than German-speakers do. But an important factor is economic openness. Germany is a trade powerhouse, so its language will be more economically valuable for an outsider than the language of a relatively more closed economy.

But in American context (the one Mr Saiz studied), the more important factor is probably supply, not demand, of speakers of a given language. Non-Latino Americans might study Spanish because they hear and see so much of it spoken in their country. But that might be the best reason not to study the language, from a purely economic point of view. A non-native learner of Spanish will have a hard time competing with a fluent native bilingual for a job requiring both languages. Indeed, Mr Saiz found worse returns for Spanish study in states with a larger share of Hispanics. Better to learn a language in high demand, but short supply—one reason, no doubt, ambitious American parents are steering their children towards Mandarin. The drop-off in recent years in the American study of German might be another reason for young people to hit the Bücher.