I have already mastered English (well, it is my native tongue after all), French is my second language, Spanish is my third language, and Sign Language is kinda my fourth language (I only say that because I am still learning).
I think that everyone should have a firm grasp on at least one language other than their native, but that would just be wishful thinking, now wouldn't it?
Well, this post is really to plug learning a language. Learn one! If not with Rosetta Stone, then some other language tool (I have used Byki, which has helped me retain a lot of information with the various spoken languages), and for sign language, I use an on-line tool which has animations and pictures that make sure that you can assess the word as good as possible.
There are also language blogs out there to help as well!
Transparent language has great blogs with various languages (sans sign language), and they have great activities as well. http://www.transparent.com/french/
For sign language, I use http://www.lifeprint.com/ and it has been really, really helpful.
I suggest you get up and start learning! It can enhance your life greatly!