What is "good for one's brain" (apart from proper nutrition and absence of concussions) is a strong education and healthy lifelong social interactions. Human language is essential for these interactions. Having multiple human languages opens more books, interactions, and cultures.

The opposite is to remain closed. This is a dangerous state of mind and culture.

From TFA:

     all these studies take for granted the uncontroversial mental superpower that you get from language study: being able to talk to people you could not have otherwise.

Not just to talk to people, but to unlock an understanding of their culture and perspectives.

Talking to more people in more contexts is a practical affordance: having more tools in the shop means being able to handle new and different types of problems effectively. People solve problems working together with people.

Having the cognitive adaptability to use new and different tools is certainly a valuable quality. We can nurture it as a learning objective, but it may may not be as universalisable as we have hoped. That said, the cost of not trying to educate people is to fail even worse.