Technology

I've lived in The Netherlands and they're "complicated" if you're used to, for example, English-style of politeness or even Mediterranean-style exuberance.

They tend to be very direct, objective-oriented and seemingly cold/closed towards strangers (they open up more with friends and family), so for example if you're in a work environment and one person's trying to do things in broken Dutch is hindering the actual accomplishment of the work objectives (for example, in a work meeting), that will probably be pointed out to them, though I've never seen it done so rudelly.

They also tend to be pretty proud of their English-language speaking abilities and when you're just learning Dutch and try to speak to them in it, often switch to English when they spot (from the accent) that somebody comes from an English-speaking country (so for me, who am Portuguese, they didn't tend to do it and I could just silently ignore it when they did because they couldn't be sure I actually knew English, but I had friends and colleagues over there from Britain, US and Australia who constantly got that and for whom it was a lot harder to learn the local language), though I don't think that applies in your example.

It bet that happenned in a professional environment or some kind of professional situation.

That said, that specific telling off would be considered rude even in Dutch terms: if a person's attempts at using Dutch are hindering doing the work, one is supposed to tell them that as the reason to switch to English (say, "other people are waiting behind you in the queue" or "we don't have time to do this meeting in Dutch", though one will probably not get a "I'm sorry but" or "I'm afraid that" or other such decorations to soften the blow which you would get in most other countries. In that quote of yours the other person making it about themselves "I'm not your Dutch teacher" and just bossing the other person "we will"(!), would be considered rude even by Dutch standards IMHO.

Personally (and note that I lived over 8 years in the Netherlands and do speak the language), had somebody told me off like that my reaction would probably be to not give a shit and carry on speaking Dutch since that person made it about themselves and I'm just as entitled to do it the way I see fit as they are to do it their way and I very much suspect (can't be totally sure) this reaction comes from that part of me that are the elements of the Dutch mindset I've taken in from having lived there so long (certainly the whole "I'm just as entitled to my preferences as you to yours" feels very Dutch).

During the period when I was starting to learn Dutch on various occasions the other person switched to English (probably because my Dutch was really bad or I was having trouble following them) and I just kept on speaking Dutch, and I think I was once or twice told off for trying to say something complex with my really broken Dutch whilst buying something and I was holding the queue, but they simply pointed out I was holding the queue.