Well, here is a list of reasons to learn English:
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If you know English well, an incredible amount of resources becomes accessible: books, courses, videos and of course, a great part of the Internet.
Most of the blogs and websites are in English (second place is Chinese).
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Many positions require dealing with clients, service providers or colleagues from other countries. In those cases, your knowledge of English is a critical advantage. It can help you advance further in your career.
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You can speak with confidence and get your message across clearly, instead of hesitating and stammering.
Even in non English speaking countries, it can be very helpful for a tourist to know English.
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You can do that while you travel to a different country or even from home using the Internet!
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You can watch movies, read books and listen to music in its original and untranslated form, which is in most cases more authentic. And songs tend to be better when you can truly understand them.