Setting: (not that it matters much): medieval England and France
If a child was taken from England at the age of five or so, and grew up thereafter in a completely French-speaking environment, am I right in thinking he would learn to speak French like a native?
How much English, if any, would he be likely to retain if he really didn't encounter any English-speakers again till he was an adult? If he retained any English, would he retain his English accent or would he sound like a Frenchman?
I'm aware that these things are a continuum, and that there will be people at both extreme ends of the continuum, either retaining the whole of their cradle tongue, or forgetting it totally. But what would be typical?