在这个世界上你只是一个人,但在某个人眼中就是整个世界．

You are only a human being on earth, but the whole world is in one's eyes

You are the one in the world, but you are the whole world in someone's eyes.

to the world you are the one,but to some one you are the whole world.
貌似是用来写情书的吧？呵呵。。。
如果是的话你可以这样说：
to the world you are the one,but to me you are the whole world.