Hi,
True what the other ladies said but sometimes under different circumstances, its easier said than done!
With our 1st who is now 6, I always spoke english to her .Hubby because he wasn't with her as much as me, he would speak english instead of french so that she would understand and interact. Holidays would be full french immersion but she would still only speak and answer in english but she understood everything. The language spoken in the country you live, also plays a part. They tend to answer and feel more comfortable speaking the language which they hear the most on a day to day basis. We've always been in english speaking countries and I'm english so for her, its normal to speak english.
She only started to speak french after 4years, although she understood everything, she preferred to answer in english.Now she switches from french to english, but will only speak english to her sister?? With our 2nd, I speak english and hubby speaks french to her, and then in english for her to understand.
She'll be starting preschool next week and will be one day eng, one day mandarin, so we'll see what happens.
Even though they look like they don't understand , they do. Just go with what you as a parent feel comfortable with and I'm sure he'll take it all in.