My daughter is 2 years old. She just turned 2 and is able to say so in two languages. When asked "How old are you?" She says "I'm two." When asked "何歳ですか” she says "２歳.” Neither my wife or I have any idea how our daughter is able to keep the two languages separate or respond appropriately but she does. It is amazing for us to watch.
My wife speaks Japanese and I speak English with my daughter. My daughter is learning both languages and can speak and understand them almost equally. She is just starting nursery school so she will start to pick up more, and more Japanese. This cannot be avoided. I don't think it is a bad thing. Every month I plan to write about our bilingual journey and talk about the things we do to help her learn two languages. I hope you can find something here to help your child with his or her language study. For today I would like to very briefly focus on the importance of routine, and our bedtime routine.
I often read the children's book "Good night Moon," written by Margaret Wise Brown to my daughter before bed. It is a well-known classic book. It is a book about saying goodnight to everything in the room and the Moon before going to bed. So we imitate this. Every night before she goes to bed we go around the room saying goodnight to the same objects. For example good night TV, goodnight books, goodnight toys etc. Every night we do this and every so often I increase the amount of vocabulary and grammar. When we first started we were saying good night table goodnight chair. Then after some weeks we practiced saying goodnight Mama's table goodnight Mama's chair. Then after weeks and weeks of saying this we now say "whose table is this?" "whose chair is this?" From doing this repeatedly my daughter now knows the vocabulary and how to use it in a sentence and is also able to answer questions.
When's a basic routine has been learned it is easier to bring new parts to it. For example increasing vocabulary asking questions comes very naturally to the child after a routine has been established. If you're teaching your child language at home I suggest finding a routine and sticking to it. Do it over and over again and slowly changing it a little by little after you feel that they have mastered it. I hope this was helpful. Good luck on your language journey!