How does learning a foreign language activate parts of the brain you don't use?

Seriously, in my opinion before I learned anything like Spanish, German, or even if it was Japanese, I used all sorts of my brain. I played Pokemon, played Go Fish, and liked Astronomy. I was into Astronomy before I was taught any foreign languages. I learned about a planet's rotations and that got me into thinking what caused the day and night. So learning a foreign language helps me think more in languages? Basically, you have to move your mouth in a different way and say a lot of different words and when you type on the internet you have to type a lot of keys or even some to get you in trouble. I speak English, German, and Japanese and I'm getting a better dictionary source so I can continue with the German although my Japanese can pretty much say all the stuff for me that I would in English or German or any other language.