"This chapter is in English."

That's a very hard question. In Dutch we tend to use articles when we speak of languages - but I don't really know why. Perhaps there is no reason.

I did find this document broadly concerning the matter of (not) using articles in Dutch and English: http://www.reocities.com/Southbeach/plateau/6805/Lidwoorden.doc But it's mainly a list of various instances when the languages differ. Do note it's in Dutch. Considering your well-written question you'll probably manage - but otherwise don't hesitate to ask :)