This is a very interesting topic. I had never thought about the fact that being able to switch easily from one activity to another would be an advantage for languages learners. But it makes sense.
What I had thought is that learning another language (when being an adult) may help improve your inteligence (at least to certain degree). It makes sense as well since it seems to me that it requires you to use your brain in a different way/s than usual, making your brain capability improve.
And as you say, since it is also a matter of attention, it sharpens it too, I think.
Everything we work on improves, one way or another!