A lot of research has shown benefits to being bilingual (here, here, and here). Now research suggests that knowing even more languages (multilingualism) may be even better for the brain's flexibility or "neural plasticity". From Science Daily:
EEG recordings prove learning foreign languages can sharpen our minds
Scientists from the Higher School of Economics (HSE) together with colleagues from the University of Helsinki have discovered that learning foreign languages enhances the our brain's elasticity and its ability to code information. The more foreign languages we learn, the more effectively our brain reacts and processes the data accumulated in the course of learning.
Researchers carried out experiments where the brain's electrical activity was measured with EEG (electroencephalography). Twenty-two students in total (10 male and 12 female) participated in the investigation, with the average age being 24. The subjects had electrodes placed on their heads and then listened to recordings of different words in their native language, as well in foreign languages, both known and completely unknown by the subjects. When the known or unknown words popped up, changes in the brain's activity were tracked.... Apparently, the ability of the brain to quickly process information depends on one's "linguistic anamneses."
The experiment has shown that the brain's electrical activity of those participants who had already known some foreign languages, was higher. The author of the study, Yuriy Shtyrov commented that the more languages someone mastered, the faster the neuron network coding the information on the new words was formed. Consequently, this new data stimulates the brain's physiology: loading the mind with more knowledge boosts its elasticity.