I am having the same problem with my 3.5 year old son. We have been speaking English in the home while living in Turkey. Now we are moving back to America so my husband is trying to get him to speak back in Turkish. He understands everything but we not respond in Turkish since he is used to responding in English to my husband. How can we insist they speak back in the same language without frustrating them and making them resent the language?  I want him to enjoy speaking it and not feel like it is something he is "forced" to do. Any ideas?