Brussels is increasingly English-speaking in general, and folks who work in the train areas in particular have to be fully fluent in French, Dutch and English, where the latter is just the default communication choice if you detect someone is not a native Dutch/French speaker. As well the area around the Noordstation is particularly unwise for a tourist to visit for cafés, so it is extra important for them to get across where you ought to go.

An amusing version of this happened to me in Porto recently at a bookshop. Many moons ago I was fluent in Brazilian Portuguese, but today that's retained mostly in reading, I can't speak well nor can I easily follow European Portuguese speakers. I was browsing the Portuguese literature section when the shop keeper asked me question (which I didn't catch), and then upon ascertaining I spoke English she rather forcefully explained where the English section was, but very confusedly watched as I politely kept where I was. To make matters probably more confusing I ended up buying a linguistically avant-garde book that I soon learned really is best suited for native speakers (Guimarães Rosa, the author, is basically a Brazilian James Joyce)

Maybe it's a cultural difference but I have a hard time thinking that this is impolite. I'd have do the same and it wouldn't mean that your french is not perfect but only that I would like to ease your life.

Most likely he responded in English because he felt it would be easier for both of you, not because he wanted to humiliate you.

On the receiving side it comes off as "just stop trying and buy something"

Which is really quite reasonable if you are at the front of a queue of people waiting to buy something, or if the cashier has something else to do like make coffee for the people who already ordered. The intent is very unlikely to be to humiliate the foreigner.

My guess is that you didn't say "bonjour" first. Major faux pas.

I'm an American living in Japan and the this happens quite a lot - even among friends who I'm pretty sure mean no harm.

It seems like a lot of people have this very strong "foreigner = English" mapping in their head. As soon as you're outed as "foreigner" for any reason, you get the English (even if you're proficient in the local language).

I definitely think it's disrespectful to switch languages on someone like that, but a lot of people just don't seem to think about it. Multi-lingual conversation manners are not commonly talked about!