grumppa- you comment is so simplistic, and I have to say an emphatic NO- the fact they were speaking English initially does NOT mean at all that English was their first language- at all. The language chosen at any given time will depend on lots of things, the mix of the group and the context. I had many French friends in the UK and we always jumped from one language to the other depending on context, and also who else was in the group. Although our children and DHs would undeerstand and speak French well, we may well speak English together when they were in the room, but switch to French when they were not. And not consciously either- it just happens naturally.
And of course so many words do not translate at all- so would always be thrown in the mix. We could speak French together but still say bits in English, like 'toad in the hole' for the dinner that night, or kids studying for GCSEs, etc- because there is not equivalent in French. When you are naturally and fully bilingual, this happens all the time- and there is no conscious decision about it, or malice, or intended exclusion- at all. I speak read, listen to French and English all the time, and switch from one minute to the next. Again, not boasting- just a way of life. Often watch the BBC, whilst reading a book in French, picking the phone and speaking in French- then ask a question to OH in English for caller, reply to him/her in French- etc. If you asked me which language at I just spoken- I wouldn't have a clue which.
The reason so many of our immigrant children were such brilliant linguists- as they heard/understood/spoke 2 or 3 languages from birth - and are not fixated on one word for each thing, and one pattern (grammar), etc.