hello Mudassar,

the reason english is known as the "international language"  goes to the British empire. You see, the Brits were known for "civilizing" the local people, and in those efforts they hoped that those people can actually learn english, and so the language now known as english spreaded far. Also, not only english was quite a bit easier to learn as a foreign language, but also it was used so much that people started using it as the official "international language"

hope it helps

Justin