Madonna is learning to speak her adopted son's native language.The 'Hung Up' singer and her director husband Guy Ritchie have been taking lessons in Chichewa, which is spoken in the African country of Malawi where baby David Banda was born. A source said: "Madonna and Guy use the few words they have learned - Mummy, Daddy, hello, my love and some others - every day."Madonna knows a Chichewa lullaby called 'Gona Mana' and has been working on a guitar piece to go along with it."The source added: "David is a very smart little chap who idolises his brother Rocco."Meanwhile, Madonna's father-in-law has claimed that her desire to adopt another child is putting strain on her marriage to his son. Guy's father, John Ritchie, said: "They've had a difficult time. David is very beautiful and Guy is very good with him. But I don't think he'd want to adopt again."They were both under an unbelievable amount of pressure because of all the press surrounding David's adoption. It has caused tension and been deeply upsetting."It has been suggested that Madonna plans to give a home to a baby girl also from Malawi.