More to the point: Speaking multiple languages expands your ability to express and describe everything, which leads to having more variety in your lines of thought. Different languages have different ways of thinking, thus speaking multiple languages nurtures an open mind.
Source: Me, native English and Japanese speaker.
For example, I remember after finding out what "tsukkomi" and "boke" are in Manzai I started seeing it play itself out in English conversations with my friends. It's kind of weird seeing a concept play out that was there the whole time, but you re-shaped your thinking to observe and categorize it.
Anecdata: I'm a native English speaker but as a little kid I was completely bilingual in French (dad was a USAF officer stationed at a NATO base in rural France; my sisters and I did our first few years of school in the local village schools). I've long had the impression that native fluency in both languages had benefits; it certainly gave me easy A+ grades in high-school and college French classes, but otherwise I couldn't begin to articulate what those benefits might be.
These days my French is pretty rudimentary, but my wife and I like to watch some English-subtitled French TV series on Netflix and Acorn, e.g., Call My Agent, Candice Renoir, and Munch; it's fun to recognize idioms and sometimes not even need the subtitles.