The difference between say the French and the Germans (let alone say, the Finns and the Greeks, or the Portuguese and Lithuanians) is literally on every conceivable level. Brits have more in common with Anglo-Saxon countries like the US and Australia than they do with say, the Belgians across the channel. This is one of the great pleasures of the continent- that in so small a space you get so many different cultures to dip in and out of, even in short distances.
The minute you walk into France, 25 miles away, it is very, very foreign. You don't understand a word and find you can't even read/ pronounce them, the food's completely different (with meals at different times), the buildings are in a different vernacular and materials, with completely different histories, the people look and dress different, read and watch media in a completely different cultural sphere, their immigrant communities source from different regions than back home, they even move differently at a different pace. The money's different, shops different, they drive on the wrong side and there's not a single brand you recognise unless it's a luxury fashion label. Weather's warmer, the flora more Mediterranean, the place even smells different. And don't get me started on the bathrooms, and how confusing that gets (seriously France, what the hell do you do in there?). Even the small things such as fonts, dog breeds, expressions, or how they pay, or what they find funny.
I mean if this feels foreign to a North American, it also does to say a Brit or German next door:
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But also note how many of these border towns have aspects of the country next door (architecturally) due to border changes - Menton looks like it has a lot of Spanish vernacular, Colmar German, Annecy Italian, Chamonix Swiss, Mont St Michel English, Carantec Welsh (Bretons are Celtic), but with a French twist.