This is an article I have had to write for my English classes as homework. So, instead of throwing it to the trash after it gets reviewed tomorrow by my teacher, I'm posting it here. Nothing specially interesting, but it may change your mind about English ;-)
These days it is becoming more and more important to know English. There are multiple reasons to make this statement, which I'll discuss below.
Let's start with (maybe) the most relevant reason. English is spoken almost everywhere, being North America (in special, the United States) and the United Kingdom the main zones which speak it. If I say "main zones" is because they have derived in two dialects of English, known as American English and British English, respectively. If that doesn't mean they are important, what else could?
These two regions are very important in the world's economy (and culture, when talking about America), which means that you'll eventually have the need to talk to people in these countries; learning English is a must to be successful. Of course you can surely find people that knows another language, but... will it be yours? This dramatically reduces the amount of people that can understand you, depending on which language you want to use.
Another reason, related to the previous one, is the following: if you make the effort to use English when talking to native English people, they'll be more receptive to what you say. That is, they won't need to do any extra effort (if you speak correctly) to understand what you want to say (which, in the business world can be important).
And the last point I'm going to make is about computer science, as it's the world I'm familiar with (although it can be extended to many other branches of science). Most of the progress in this area happens in the countries I outlined above. But even if it happens in other countries, people tends to use English in their own programs, articles, or anything they write just because they will get a bigger audience.