When my son and I returned to the UK, he could only speak Thai and some basic English. So, I spoke mainly Thai with him to make the nipper feel comfortable. After six months or so at Primary school and being surrounded by English in everyday life, he could speak it quite well. In fact, when I said some words to him in Thai, that he understood when in Thailand, he would give me a puzzled look to say he no longer understood them. Sometimes, when I tried to speak to him in Thai, he'd get angry and insist that I spoke English.
When his mum arrrived here a year after we had returned, she was surprised at how well he could speak English. She even has to ask me on occasion to translate some of the things he says, because she doesn't understand. She was NOT happy that he could no longer speak Thai, and now speaks to him in a mixture of Thai and English - Thailish.