The mind of the polyglot is a very particular thing, and scientists are only beginning to look closely at how acquiring a second language influences learning, behavior and the very structure of the brain itself.
When we learn another language the structure of our brain is changed. That change leads to a greater efficiency of use in the brain. The brain is better organized and uses it’s energy in a more conservative way in comparison to a monolingual brain. Studies also suggest that multi-(bilingualism) boosts the so-called executive function of the brain. This command system is responsible for planing, solving problems and performing tasks. In order to stay focused, distractions have to be ignored and important information must be kept in mind.
Because polyglots have two or more simultaneously active language systems, they perform better in multitasking since that is what they do everyday, organizing their languages. Due to that they also need less brain activity (energy) when performing the same tasks as monolinguals.
This change happens differently depending on what age the seconde language is acquired but it occurs to some extend no matter when another language is learned!!!
A second, less scientific way multilingualism makes you smarter is through literature. When you can speak multiple languages you can also research in linguistically diverse sources. Sometimes important things might not be mentioned in your country but are in others.
It is also possible that different languages lay-out topics in a different way and when you understand them you can determine the complexity of the subject.
So let’s leave monolingualism far behind us! (>_<)