What causes so much concerns to unilingual Americans, is in fact a no-brainer (pun intended) to most of the rest of us out here in the world.
In Europe, unilingual people are hard to find. For Belgians of my generation, we were taught Dutch, French, English, and German at school (and Latin and Classical Greek), while (in my case) an absolutely beautiful local patois of Flemish was the common vehicle for non-formal conversation.
I won't list the other languages that have crossed my path since my school days.
In India, almost everyone I know speaks three to four languages. A speaker of Kannada, for instance, will have no trouble shifting to Tamizh, or (perhaps with somewhat less enthusiasm) be able to address you in Hindi or English.
In Singapore, where I live now, there is English and Singlish, and three more official languages, of which everyone takes at least one.
My 15 year old daughter is a native speaker of English - plus she is good in Flemish, Dutch, French, and Manadarin - and has strong basics of Latin and Spanish.
Basically, we don't need to study the brain to know that multilinguality is good for us and helpful for society. From my perspective, American exclusivism and other forms of detrimental one-language nationalisms have caused the world enough miseries.
Languages open our worlds and our minds.