We mixed it up. Sometimes speaking Thai sometimes English.
When I first met my, now, wife she didn't speak English. My Thai was OK but not great. We learnt each other's language from each other and continued learning when our daughter was born by using both languages at home. I learnt to read and write Thai at the same time as my daughter, with kids books.
We lived very close to my wife's family. If they were present we always spoke Thai. I think it would have been, still is when we are there, impolite to speak in English within their earshot.