Aren't there lateral benefits to learning something as complex as a new language? The level of mental focus and commitment required must have cognitive rewards and I would think any level of cognitive improvement would be of great value.

In order to learn any language, it requires a certain level of immersion in cultural concepts/perspectives outside of your own. Broadening cultural awareness and gaining new perspectives certainly contributes to an individual's ability to see the world with increased clarity.

It seems to me that measuring the worth of learning anything in terms of how directly one might make use of it cannot measure its total value.