I have taken Spanish, French, and German. I believe that learning a second language makes you more articulate in your native tongue. It gives you a better grasp of how grammar and vocabulary function in relation to themselves and also increases your own vocabulary.
Having said that, the only real con to learning another language is that sometimes you flip into it by accident and no one understands you. If you learn more than one language of the same type, like Chinese and Japanese, or French and Italian or Spanish, the words get jumbled up. The words for things in spanish and french and italian are often very very similar. it can be difficult to keep the lists of words separate from one another.