My biggest culture shock was in Belgium, I lived there for a few months for work. My boss was also British and stopped for petrol but it had a weird system, paying with card first or something. Someone came running out of the shop bit and called 'English? French? Flemish? German'. He apparently spoke all of them and wanted to check but it always struck me that he asked in English. Similarly when I worked in Paris, my French boss and I were in Florence in a gardens and she automatically spoke in English to the Italian person on the gate. Neither had English as first language but it was the done thing to use it.
Another nanny I worked with in Paris was so grateful I was there, as a native English speaker, to help her keep her language skills (learnt from her own nanny as a child). I tried a couple of times to get her to converse at least a bit in French but nope, she was desperate to keep up her skills. Apparently she had many more job offers, could ask more salary, was so much more employable as a fluent English speaker.
I always took it for granted that I spoke English, but it opens your eyes to see just how useful that is. In the end I learnt very very little French, my job was to speak only in English to the toddler and I was there 6 days a week. My French boyfriend would make faces at my appalling accent and attempts at speaking French so I gave up!
I did find Paris to be quite rude but no more than some places I've been in in the UK. A couple of places were happy you tried to maybe ask for a bag in French or whatever but most shop workers would just carry on. It was the same in French speaking Belgium, around Brussels, but where the family I was with actually lived it was Flemish speaking and my attempts to say even thankyou in shops would get grins and perfect English in reply!
Another thing in France that struck me is the difference in milk. In a UK supermarket most of an aisle is fresh milk, there every supermarket I went into had at most 2 types of fresh milk, so only 2 spaces in the fridge, then a whole aisle that milk in cartons you don't refrigerate. But then so much yoghurt!
Paris was also weird to me, not sure if this is all of France or just there, but random doctors all over the place. When I took the toddler to the doctor it was just a random apartment off of a courtyard with others that were homes. Just a small waiting room then the one doctors office. Then going with the mum to a different doctor, on the other side of the city but again one doctor just working in a flat. Like a proper dr office, with all the equipment and decor and that smell you recognise, but not a clinic or anything.
This one may have been the family I worked for but it shocked me the amount of cosmetics etc they used for the toddler! He was just turned 2 when I started, we had a cleanser for his face, little capsules you split open to wipe his eyes. A perfume to spray on him, cream for around his eyes and a gel on his face. And this horrendous contraption for sucking snot, I think called a moushe bebe or something like that. It involved a parent holding him flat on his back and the other putting a tube up his nose and sucking a weird mouthpiece- snot ended in a separate tube not the mouth! But he screamed and cried and was so afraid, I told them I'd never use it on him. No need.
On holiday in New York the biggest cultural shock I had, I was kind of expecting- the patriotism. The sheer amount of flags on buildings, windows, all over. It's an odd thing but seeing an English flag flying here is kind of a bad thing, outside of sporting things!