Do we have to learn english?

if answer is "yes", why?

or

even though the most spoken languages is spanish, arabic, english, turkish, chinese. why do we speak spanish or arabic ..etc instead of english?

Well, my own language is spanish and I thought that is full of grammars and different kinds of modisms that we have made up the time we have developed our language and English is a flexible language that most of the people learn without getting into troubles with the grammar stuff.