How can I encourage my child to learn a mother tongue which we don't normally use at home!!

Someone in the house have to use that mother tongue and communicate with that child often. Even if its a one way communication, continue with it. Try to get other family members to cooperate with you. Soon your child will get the hang of it. My son refuse to speak chinese at home despite all of us speaking chinese to him. He rather interact english with us. Which made us use english more often. Aft having a china teacher taking over his class, using no other lanugage but chinese to communicate, now my son is able to tell us about his day in chinese too!