I’m not dumb enough to make that mistake! Regional pride is a serious thing the world over. From and outside perspective, though, I could not detect much difference in the language (accents) and culture. Some, to be sure, but easy enough to fit in either place.
I think that depends a lot on which part of the U.S. you are talking about - Canadians don’t tend to have a lot in common with Texans - but have a lot in common with people from Minnesota or Seattle.
But then, Minnesotans don’t necessarily have a lot in common with Texans, either.
(We went on a cruise this year and my daughter hung out in the teen club - which segregated itself into primarily two contingents - Texans and Canadians. My Minnesotan daughter was accepted into the Canadian contingent - but most of the American kids ended up with the Texans.)
I think most Scottish and Irish accents sound pretty different to us, but I lost track of the number of times people in the US asked me what part of Ireland I was from.
Of course, that’s probably just lack of familiarity with a couple of relatively rare accents. I imagine most Americans who spent any time in either country would soon learn to tell the difference in most cases.
It seemed to me that Flanders (northern half of Belgium) and the Netherlands were just about indistinguishable. IIRC, Flemish and Dutch are essentially the same- on par with the differences between US and UK English.
I’ve heard them described as one people separated by history, what with Britain having taken over Malaya and the Netherlands Indonesia. I once attended a conference in Yogyakarta on Java that had both Indonesian and Malaysian participants. One side said they could understand the other better than the other could understand them, but I don’t remember which side was which.
How do you factor in the Malay-Chinese, about 30 percent of the population ?
There are so many different languages being spoken in Malaysia and it is far from homogenous.
It is a very cool country and I love it, even considering doing an early retirement there.
There are ethnic Chinese in Indonesia too, although just 3-4% of the population. Indonesia really clamped down on their Chinese after the fall of mainland China to the Communists. It’s only recently that Indonesia’s ban on Chinese script was lifted … at least, I think it’s been lifted. The Chinese are a more recent arrival in this neck of the woods, I think. About the 19th century – Thailand (Siam), Indonesia and Singapore. There was a big exodus at that time.
There are some good Chinese museums in Bangkok’s and Singapore’s respective Chinatowns detailing the diaspora. and ones for Peranakans (Chinese-Malays) in Singapore and Malaysia, particularly in Melaka.
True. And then there is Quebec, of course.
I’d be surprised if the dialects aren’t even closer than that - more like American and Canadian English rather than American and British, which are noticeably different. But that’s just a guess - any Dutch speakers out there who can confirm?
And what about Wallonia and France? Would someone absent-mindedly driving over the border bee able to tell much of a difference between those two?
Your first sentence is brilliant, and wins the thread, IMHO.
Serbia and Montenegro have similar history, culture, food, ethnicity, literature, religion, and music.
And you’d be hard pressed to identify more than a handful of language differences that separate the Montenegrin dialect from the Serbian language. (Although about 1/3 of Montenegro uses the Latin alphabet, and 2/3 use the Serbian Cyrillic alphabet.)
Now, your second sentence is way … WAY … off base. Three (or four, depends how you count 'em) different languages, three different dominant religions, and, BTW, a fierce hatred of one another.
Mark my words, there is a civil war brewing in the Balkans as we speak. (But one could say that at any time in the last three centuries and it would be true.)
If Serbia and Montenegro don’t win this thread, then I think it should be Flemish Belgium and the Netherlands – proposed by bump.
It’s off topic, but I’m curious - what makes you say so? Civil war in which country?
I was going to protest that if we’ll accept parts of countries, then surely Flanders and the Netherlands couldn’t be more similar than the US and Anglophone Canada. But then I recalled that you don’t even have to change money or produce a passport to go from Antwerp to Amsterdam, so you may well be right…
The problem with comparing Canada and the US from the perspective being able to “pass” in the other country is that it assumes homogeneity in both countries, which simply does not exist.
For instance, saying that Canadians generally sound like Americans ignores roughly 25% of the Canadian population, who are francophone. You can’t just ignore that 25% of Canadians likely cannot “pass” in the US for the purposes on this discussion.
And it works the other way. The US is not homogeneous in accent. My general rule is that the farther south an American comes from, the more they stand out in Canada and in some cases, the more difficult they can be to understand. So again, you can’t just say that "Americans can “pass” for Canadians.
And that’s just the language. If you start talking politics, particularly around the issues if health care and guns, some significant differences often emerge …
They will vehemently object to that (as I learned when I lived in New Zealand). Australians and New Zealanders can easily tell each other apart, even if many other people can’t. As has been said, the accents differ somewhat.
Australians and New Zealanders are a case whether either could pass for the other in a different country, but not in each other’s country.
Newfies and New Englanders stick out like a sore thumb too.
I think the Catholic Croatians, the Orthodox Serbians and the Muslim Bosnians would disagree with you.
On the topic of being called British, Irish comedian Dara O’Briain said, “We don’t like that; we fought wars.”
Yep, I was going to say this. At least till they open their mouths and start trash talking the other former Hungarian state that’s right next door. My grandfather lived his whole childhood in Croatia, but he called himself a Serb and don’t let anyone say otherwise… 
Other than that, as an U.S. American, I’ve felt a certain affinity to Australians. Can’t quite explain it, but it’s stronger than a tie I’ve felt to, say, Canadians.
Swiss German itself is a proper language distinct from German, and German speakers mostly don’t understand it. However, when the Swiss speak the German language (which they all do*), they may or may not sound like Germans (and Austrians)–I don’t know.
As several have mentioned above, the success of the “passing” depends entirely on the beholder, so the premise of this OP and thread is kind of pointless. I, for example, have a hard time telling some Iranians from some Afghans (which can offend them), but I can easily tell a Colombian from a Peruvian. And I actually have to be reminded that one of my best friends is a Canadian.
*I mean in the German-speaking part of the country.
The big question is, whom are you trying to fool?
If a brown-skinned Peruvian man named Pedro Gonzales told me he was from Cuba, I’d have no reason to doubt him. His accent MIGHT give him away to a real Peruvian or Cuban, but I’d be fooled. The same would be true of people from any number of Central or South American countries. Could I tell a Nicaraguan from an Ecuadoran? Could I tell a Dominican from a Venezuelan? Probably not. There could be LOADS of differences that a South American would spot immediately, but I’d be completely fooled.
Could a Belgian Walloon pass for a Parisian? Could a Flemish Belgian pass for Dutch? In some places, he probably could.
Could a German-speaking Swiss pass himself off as Austrian? Could an Italian-speaking Swiss pass himself off as being from Turin? In many places, sure.
If an Arab from Cairo told me he was from Morocco or Iraq, would I know the difference? Almost certainly not.
That DOESN’T mean the accents, lifestyles and cultures are identical- just that, like many outsiders, I wouldn’t see the differences right away.
Several of the answers (mine included) demonstrate an ambiguity in the OP: is “passing” meant to be in each other’s country, or in a third country? The more different the third country is from the proposed pair of countries (in location language culture, etc), the easier it would be to pass.
But it seems the best measure is whether it is possible for citizens of country A to pass as citizens of country B, and vice versa.
In other words, is the OP asking which two countries look most similar to outsiders from both, or to citizens of the two countries?
The posts by Colibri and guizot illustrate this point very well.