I'm not OP, but the answer is obviously both. Multilingual helps brain develop in new ways, and better personal development helps all nations.

I sometimes wish my childhood environment were conducive to holding onto a second language. I took Spanish in high school but because there was nobody around to speak it with I lost it entirely.

I’m now working on acquiring a second language I might actually get some usage out of, but nothing can parallel learning secondary languages early on.

> Multilingual helps brain develop in new ways

I've never seen conclusive evidence to this.

From an evolutionary perspective it makes no sense either. Multilingualism would happen sometimes, but it was only when your tribe was conquered by a neighbor. Other multi lingual scenarios are very recent. If there are any evolutionarily rooted features around this, it would be related to the former. Though, there could still be some coincidental benefit, it just seems as unlikely as any other totally coincidental benefit.

Do you eat tacos, sushi, omelettes, or fettuccine? Do you practice kanban, yoga, zen? Where do you think these words come from?

English is a foreign language to the Americas; would you say that it’s a net bad to have it be spoken here (and be an official language of the country?)

I chuckle every time someone uses the "everyone in America is a foreigner" angle as if it helps their position on supporting mass immigration.

Mass immigration didn't work out for the Native Americans. Why use that as a model to replicate?

in the most negative view, your nation having speakers of foreign languages is good for having a healthy population of potential spies that you can send out to foreign nations.

Everyone.

The wellness of the nation can be taken into consideration once that nation takes the wellness of its people into consideration.

Both, easily. I wish I could give my kids that advantage.

Worked for Switzerland.