Mahmud was a very pious man, but he was not very attractive looking. One day, he was walking along the road, minding his own business, when a lady came up to him, and started calling him names and abusing him. This lady kept saying all kinds of nasty things, and Mahmud just listened to her patiently. Finally, when she was finished calling him names and abusing him, Mahmud just greeted her, smiled, and said
"and a nice day to you," and continued on his way.
Why did Mahmud, the pious man, behave this way?
It is because he knew that everything she said was just a reflection of her own inner self.
If you use bad language, you are just making obvious to everybody how far away you really are from Allah, since your words are in reality a reflection of your own inner self.