For many, being able to speak the world's most widely spoken language means that they don't see any point in learning another. It's true that people in most countries in the world know how to speak and can understand English, but there are SO many reason to learn how to speak a foreign language.
First, it's actually way easier than you would think! Learning a language doesn't have to be difficult, even if you're not a natural linguist. The Internet has some great sites that can help, with "user-friendly" games for learning new vocab, and clearly explained grammar rules. YouTube also has some simple videos with step-by-step examples to help you learn a new language.
Second, if you travel to a foreign country and speak to people in their own language, you get RESPECT. Locals always love when tourists make an effort with getting to know their culture, and even something as simple as knowing/asking how to say thank you can make their day. Some simple vocab knowledge is also helpful should you find yourself lost in a foreign city, surrounded by locals who don't speak any English...
This might be nerdy, but whatever: Knowing a bit about another language can make you understand more about your own. Here's something I find super cool: The ancient Greeks gave what we now know as the giraffe the name 'camel-leopard,' because to them, giraffes look like a cross between the two.
Finally, being able to speak another language means you can communicate with people from other countries when there would otherwise be a total language barrier. I have friends across Europe, and it's always really interesting sharing ideas with them, and seeing how where we live affects who we are as people. If I was not able to speak other languages, I wouldn't be able to have that insight into other cultures and other counties.
So you want to learn a new language? First, pick the one that interests you the most. Then, check out three tips for subtly improving without even realizing!
1. Get a pen -- or Facebook -- pal
Okay, so no one writes letters anymore. So find a friend-of-a-friend who speaks the language you're learning and add them on Facebook/BBM/Whatsapp/Skype. If they're your own age and share some similar interests, you're not going to mind speaking to them once in a while. The way they misuse English can sometimes be really entertaining (they'll be laughing at your Spanish/French/Italian/German etc. too, of course, but don't think about that...).
2. Read magazines in a foreign language
Reading magazines is so much more fun than reading novels, and it's easier (obviously), since the articles are in small chunks. It's also useful for picking up some more colloquial words that you wouldn't get in a textbook -- particularly if it's a magazine especially for teenagers. You can also decide which articles you read based on what interests you, and you can pick the magazine up every now and again when you have a free moment...
3. Watch foreign language movies (with subtitles)
Everyone likes watching movies (especially if they involve topless Spanish boys), and even with subtitles, you still absorb some words and phrases. It helps a lot for your spoken language, and you look really smart (and not nerdy AT ALL) if you casually drop into conversations that you've seen loads of foreign-language movies.