It's the language that everyone in the whole world knows
a international language is a language which is all most know by every one in the world and it is used by every one is their day to day life. for all of us English is a international language and we use it every day. the international language is know by every one and it is even taught for every one because when some one wants to interact with some one in a international country or city to communicate he/she has to know English so that they interact easily.  

hope my answer is useful and helpful................