Less well known languages like Japanese tend to have higher returns on investment than common ones like Spanish. IIRC Spanish has a return on investment of about 3%.

In my experience the Japanese translation market was very crowded and didn't pay well so my first plan went out the window.

I forget if it was Planet Money or Freakonomics, but there was a podcast about the ROI of learning a foreign language and according to the research they cited the only foreign language worth learning from a pure ROI perspective was English.