Oh absolutely, and of course using simplified language is what a sensitive and responsive teacher will naturally do. When my son's teacher is speaking directly to him, she uses a simpler form of French than she does with the others - easier words, single clause sentences, slower delivery. Naturally I'm delighted at how she's helping him progress. Totally hypocritically, though, I would be concerned if a teacher was delivering all the lessons in a similar semi-basic English in an English-speaking school that my kids were attending!
But 'concerned' in this context could simply mean being more aware of the English we used at home, to make sure it was fully enriching, included more poetry reading, TV viewing was more carefully screened, etc. I was just flagging it up as something to be aware of.
Before arriving here, I too saw this as a huge potential benefit of the international system. I was surprised to discover just how varied the children are in my daughter's class: Swiss, English (just her), French, German, Albanian, Ukranian, Chinese, Portuguese - and that's just the ones I know personally! Obviously this won't be the same for all areas (we're along the shore of Lake Geneva) and this too will create issues of making the teaching language accessible for all the children. Luckily, although many of the others have different 'home' languages, my daughter was the only totally non-French speaker at the start of the year.
There's always a compromise somewhere, I feel. If I had secondary aged children, I think I would have popped them into international school, no question. I have it in the background as a possible approach for their secondary education anyway, when they get to that stage.
As mine were younger when we moved over last year (8 and 4), I felt able to conduct a huge 'lab rat' experiment where in the hopefully short-term I trade off large chunks of their primary education and currently restricted socialising against bilingualism and being more settled in the community. Only time will tell if this was the right approach .
kodokan