<<Spanish was the least valuable(1.7), followed by French (2.7%). Knowledge of German, Italian, Russian and Chinese was slightly more valuable, translating into an average 4% income boost.>>
From the study you can say that learning Spanish is not beneficial in your career but what the study don't say it's that that's normal because there are more people who know the language and in consequence the firms have more bilingual people to choose from, so they pay less. Therefore if you want to get paid more, you should learn German or Russian or Chinese or any other less-speaking language but only if you would like to work with companys from those countries.
Anyway I don't see much difference in the income if you learn one or another even if you don't learn any of them.