Hi bearmay,
We are also a mixed couple with me being Chinese and my husband being Spanish. We live in Hong Kong. At first we spoke to her in our mother tongue (i.e. Cantonese + Catalan) and our maid would speak to her in English. We believed that exposure in different languages would do her good in her future development. It was also our pride and ego that we wanted her to learn our mother tongue.
However at the age of 2 years and 2 months, she could say very few words, less than 10 out of her initiative. She understood more words but still behind kids of her age. Communicating with her was also difficult because of her delay in comprehension and speech development. We found her difficult to raise. She simply wouldn't take instructions (probably she didn't understand). We never knew what she wanted as she would only cry and cry.
We had put her in playgroups since she was 9 months so she could spend more time with other children. We knew our girl was active and late in talking but we knew we shouldn't compare her with other kids. We thought that given time and patience she would be better. We tried to look for pre-nursery classes to her to give her even more exposure. But our application was rejected since our girl was like a wild one, didn't understand instructions, didn't speak, just kept on running around playing with different toys.
This was a turning point when we looked for professional help. We couldn't believe when we were told that our girl had comprehension and expression delay of 6-9 months, meaning she reached the level of 18 months only.
There could be many reasons causing the delay and we guessed/concluded that language confusion, insufficient stimulation, active character -> seldom stops to listen, too much TV exposure -> tend to reduce attention span, amongst which language confusion is the largest suspect. We had ruled out hearing impairment after conducting hearing tests. At the advice of almost everybody, i.e. social worker, school principal, paediatrician, speech therapist, we switched our "official language" at home to be English. On top of that we gave her intensive home training to catch up on her cognitive development (back then she didn't even know that an apple is called "apple"). We also took her to speech therapy sessions to get ourselves trained so that we can "train" her efficiently and effectively ourselves. Afterall, we are the ones with whom she spends most time with. Of course we also reduce TV exposure and become firmer on discipline.
Our girl is now 2 years and 9 months. It has only been 6 months but we felt like we have come a long way. Our girl can now express what she wants and the tension at home has eased a lot. She can speak short sentences of 3 to 4 elements. She is still behind kids of her age but we are very proud of her that she can make so much improvement in 6 months, especially that she really started from scratch.
Our choice of switching to one language is not met without different voices. We heard many other parents saying we should stick with 3 languages so that our kid would grow up multi-lingual. We heard whom and whom doing the same and their kids could speak 5 or 6 languages at the age of 5 only. We shouldn't waste such an opportunity of hers. We should have been more patient and waited for her language explosion at a later stage.
Honestly we had our own doubts. We also consulted the paediatrician and speech therapists whether we should follow this theory. The answer was: it all depends. It all depends on the ability of individual child. Normally a toddler can absorb 2 or even 3 languages without showing delays. However, if a toddler cannot for whatever reasons (e.g. slower in comprehension, attention deficit, ...), s/he simply cannot. We didn't want to gamble the precious time of our girl and decided to let go our ego -> mother tongue. We didn't want to find out a year or two later that her language explosion hasn't arrived yet and we had wasted so much of her time. Our plan is now to let her solidify one language first before moving to others.
Our experience does not necessarily apply to you. But it may show you different things that you need to take into considerations. All I wanted to say is it will depend on what you two think and whether your child can take several languages or not.
Thanks for reading such a long passage.