I know our child with only English and Finnish-speaking parents always preferred to reply in the local language (Finnish) for a long time. He would understand English questions and conversation, but if I said "Do you want to go outside?" the answer would be "Ei!" (no).
It took a year or so before he was consistent in replying to speakers in the appropriate language, and even then sometimes the languages would get mixed up , mid-sentence at times.
Even now I think it is apparent that Finnish is his real native language. Although his English is basically perfect there are rare lapses in grammar that make it obvious he's constructed the sentence in Finnish and translated to English rather literally.