I agree with Gwendal on most of this. But I like to add that the so called "personal preference" of mine, is merely Jawbroken's projection of me, and not actually my stated preference. However, I agree that it is irrelevant. I also like to repeat that I have not suggested any rules.
I don't understand the polarising tone in this thread. Why are people talking about "harm", or wether or not they're "offended"? I don't thing anyone is neither actively or implicitly doing "harm" by writing in a certain language, so there is no need to talk like that is the case (or like someone is suggesting that that is the case). The tone of this conversation is somewhat harsh.
I think Jeremy is asking the right question, as I understand the gist of OP, when he seeks to find a pragmatic solution to what boild down to a de-facto international audience participating in a de-facto English forum. This conversation should be about practical utility, challenges and solutions. The point of this conversation should be to make sure that the forum is as useful and welcoming as possible, to as many as possible. How can we achieve that goal?
I personally think it is hindering participation to switch language mid-thread without giving context, forking it into a new thread, or giving a short summary in English. I don't, however, think that we should impose rules on which languages are acceptable, and I don't think that anyone has caused harm, tried to exclude anyone, or be anything but helpful. In fact, I quite enjoy how much effort Jeremy put into his reply, and especially the effort he makes in formatting his replies in ways that makes it easy to read (given that you understand the language, that is)