Asking my supervisor to be frank and give feedback I am doing my PhD in the Britain and I've got a lovely supervisor which is like a friend for me. I want to ask her to give me direct feedback and tell me directly if I am doing bullshit or about any other thing that she thinks I am not good and need to improve (forexample if the way I send email is not very natural in the sense of language and ...). I want to say:

I would like to ask you to give me direct feedback on anything that you think I may need to improve. I won't be offended because I know your intention and I know you want me to be the best. I would be very happy to receive comment because I know that's what makes me improve. 

Is it good or not? Is there as better way of saying that?
It seems that you are overthinking this. You say she is like a friend. Neither a friend, nor an advisor, has any incentive to lead you into traps. Accept it if she praises your work. Perhaps you are on the right track after all. 

But we can all improve at the margins, at least. If you have specific doubts about particular things that you do, or write, ask about just those things and listen to the answers. Perhaps there are suggestions for improvement in there that aren't criticisms, but just pointers to better ways to do or say things. 

But, your note almost sounds like you are soliciting negative feedback. 

I'm pretty sure that if a friend and advisor saw you going down the wrong road you would be told that. Otherwise she is wasting her own time as well as yours. That doesn't seem like a reasonable action for an advisor. 

After all, if you finish and go on to a brilliant career it reflects well on her also. Relax.