If that's the case, the I am surrounded by a pretty intelligent population. With Basque and Spanish as official languages in the Basque Country, these people are constantly switching back and forth between the two. You don't need Basque to get by here, but anyone who speaks Basque as their first language has learned Spanish just as well too. And even the Spanish speakers who don't speak Basque can get by with some vocab and phrases.
Some time ago, researchers believed that trying to use two languages would only confuse children who were simultaneously learning two different tongues. However, recent research has shown just the opposite - that having to constantly be thinking about what you're saying is like a great exercise plan for your brain. Staying focused while doing meaneal tasks such as speaking conditions your brain to be a better problem-solver and better use functions involved in planning, doing complicated tasks and so on.
Below is an excerpt of the article which speaks to the type of tests that have been done to reach the results about bilingualism helping not hurting children:
In a 2004 study by the psychologists Ellen Bialystok and Michelle Martin-Rhee, bilingual and monolingual preschoolers were asked to sort blue circles and red squares presented on a computer screen into two digital bins — one marked with a blue square and the other marked with a red circle.
In the first task, the children had to sort the shapes by color, placing blue circles in the bin marked with the blue square and red squares in the bin marked with the red circle. Both groups did this with comparable ease. Next, the children were asked to sort by shape, which was more challenging because it required placing the images in a bin marked with a conflicting color. The bilinguals were quicker at performing this task.
Obviously starting a second language at a young age has positive affects throughout your whole lifetime, but learning one at an older age helps just as much and is said to help prevent dementia!
Throughout the world, people have taken note of the benefits of speaking more than your native tongue and bilingual schools, called immersion schools, have popped up around the globe. For example, where I work, the classes are taught in English although the students speak Basque and Spanish. They learn math, social studies and science in my language, because immersing yourself in a language has a better effect than just studying it an hour a week. Same as the in the States where your kids can study and go to school in Chinese or German.
After having lived in Europe for some time now, it kind of becomes embarassing that us Americans, for the most part, only speak 1 language. In Europe, people speak at least 2 and in some places many more. My friend Emma speaks 7! And when I say 7, I mean she speaks all 7 well. If you go to Germany or Sweden you don't even have to worry about speaking their language because everyone speaks just as good of English as you. Not only have these speakers learned in a classroom but they also watch American films in the original version, see our TV series and read books in the language that they were written. It's very impressive!
The Basques are constantly jumping between their own language and Spanish. I find myself doing the same with English and Spanish. And from that develops Spanglish that we know so well in the States. And here, I don't know what you could call it - Basquish? But if you listen to a Basque conversation you will hear quite a bit of Spanish peppered in, and if you're speaking in Spanish it's not odd to throw in a Basque connector word.
To give your brain a little workout, here is a quiz that I use with some of my private students to engage both Spanish and English at the same time. At 14 he is already on his 3rd language! It makes you wonder, if speaking 2 languages makes you smarter, what does the 3rd and 4th do?! Happy studying!