Stella pointed out to me today that Amy appears to have begun to use the appropriate language when talking to each parent, at least in a small way:
When she wants her nappy changed, she'll say ['vivi] to Stella but ['nini] to me. (And as an aside, having her being able to express this need is useful in and of itself; means less guessing what she wants or constantly asking whether she needs a new nappy.)
['vivi] sounds as if it's the first few sounds of the German word, Windel, reduplicated (a bit like how I used to call myself ['fifi] as a small child, from Philip). I'm not sure where ['nini] is from; the consonant is most likely from "nappy" (or possibly from "new", from the common phrase, "Do you need a new nappy?"), but the vowels don't match that well. Perhaps they're from ['vivi].
Still: we have established communication!