Being fluent in many languages means opportunity to open many doors. Every language has its own history and dynamics, being fluent in language means you are able to comprehend and understand someones culture, look at things the same way the other person sees the world.
I have travelled the world and I have noticed that people are more responsive towards you when you are trying to communicate in their language, you can see that they are more open, smiling to you, they are ready to communicate back to you with a bigger pleasure or even most of the cases they will look surprised that someone has put in effort to learn their language.
Deep knowledge of any language brings people closer, gives an opportunity to understand how people of that language see the world and the structure of the language can help to understand the way people think.
I have not checked any research everything is from my experience. On the other hand, just imagine if you are able to understand animals, probably in that case we would find so many interesting things what out eye is not able to detect.
