Great link. From the cited paper:

The earnings of college graduate who speak a foreign language are higher than the earnings of those who don't. Our estimates of the impact of bilingualism on earnings are relatively small (2%-3%) and compare unfavorably with recent estimates on the returns to one extra year of general schooling (8%-14%), which may help explain current second-language investment decisions of monolingual English-speakers in the United States. the returns may be higher for individuals in management and the business services occupations, and they seem to be higher ex post for individuals who learn a second language later in life. Note that the results in this paper are estimates of the gross returns to learning a second language. Individual decisions on whether to study a second language will depend on (among other things) the opportunity cost of the time devoted to learning it and its nonmonetary rewards.