I think we have mentioned before that BK1 & BK2 often use English when they're playing. Now that BK2 is starting to really speak this has only increased. The Babelwife and I try to stop them, but frankly, it's a losing battle.
So why do they use English when they play?
My guess is that BK1 spends most of her time with children who speak English and she obviously plays with them a lot. She does not play with me or BW as much. So in a play situation, the language is likely to be English. That means that whatever she says in that situation will be easily available in her head in English but not necessarily in another language.
BK1 and now BK2 basically have a huge repertoire of play-related phrases in their heads, all English.