There are lots of facts you can find which help to persuade you that learning English is a good idea - 400 million first language speakers, 700 million second or foreign language speakers, over a billion people learning it right now! And over 80% of the information stored in the world's computers is in English, more than half the world's scientific journals are in English, it's the main language on the internet, and so on*. The reason is not hard to find: nowadays it's the enormous political and economic power of the USA, though British imperialism in the last century is another important, historical reason.
The result is that more and more every day, people expect you to know some English, and it's not only English speaking people who expect it - another fact often given is that 80% of the English used in the world is used between non-native speakers. In short, it's become the main way you get around, the main way you get things done, the way you make friends, the way you do business with a foreign country, the way you get information.
If English was just a boring, practical language for getting business done - a sort of computer programming language - this growth in its use would be horrible, and we would all have to fight it. But luckily it isn't only that. It's a rich language, with many forms and varieties, ever changing to meet the needs of the modern world, but also with a rich cultural history. If you learn English well, you have access to one of the biggest and brightest bodies of literature that has ever existed, as well as to the many new examples of its power, as in films and song lyrics.
And lastly, it is a rich language. There are many things which other languages do better, but English has excellent flexibility and great richness of vocabulary - features which make it hard to learn but very satisfying when you can begin to control and use it. So even when you have some English, it's a good idea to try and get more: learning English is one activity where you can be sure you will never be wasting your time.