Could someone from Germany, Austria, or Switzerland (German speaking community) pass as Pennsylvania Dutch or vice versa? Sure, your average Austrian isn’t going to be able to “pass” as Penna Dutch in Penna Dutch country, but could he credibly fool a South African or Australian into thinking he was from Penna Dutch country?
Pennsylvania Dutch is not modern German.
Speaking of those three countries, IANA native German speaker, but even I can easily pick out a Swiss speaker. Austrians and Germans sound pretty indistinguishable to my ear, and their culture, economy, and much else are also very similar, so they are probably among the top two most similar countries in the world as far as I can tell.
It also seems to exist in parts of the west. My wife’s relatives in Calgary have very strong Canadian accents.
Yeah, but most western anglophone Canadians have generic North American accents. Yes, there are regional accents in Canada, just like there are regional accents in the US. But Canadian regional accents fall into the mishmash of northern/generic/midwestern North American accents, they aren’t nearly as distinct as the various southern US accents are.
Yes, it’s sometimes possible to spot the Canadian when they pop out with an aboot or an oot or a zed or a back bacon. But the Canadians among us can usually pass effortlessly, whereas most people from south of the Mason-Dixon line are instantly marked by their dialect.
This sounds right (to this particular American). HGTV runs a ton of Toronto-based house-hunting programming in the US, and while it’s not the full-on “aboot,” it’s definitely different enough to stand out. The station typically runs this programming without any indication of its origin, presumably thinking that American audiences will be more receptive to something that’s not marked as foreign, so it’s always been a little amusing that the word “house,” which so much of their programming revolves around, is such a marker.
Well, it’s not quite the same thing, but we could poll our South African or Australian dopers on whether the singer here is native Pennsylvania Dutch.
Canadians have a different tone to their voice too. It’s hard to explain, but the way I’d describe it is Americans have more of a “country” drawl to their voice.
Does it count if a citizen of one country could pass for someone from a specific part of another country? I may be wrong, but I have a vague sense that Bavarians may have more in common with Austrians than with people from other parts of Germany.
From my POV I think Scotland would be the easiest country for an Irish person to move to and slip into. I know lots of Scots here and I’ve met plenty of Irish over in Scotland. Cultures aren’t identical but there is a lot of common ground and history due to mass migration both ways over centuries.
How similar/different are the accents? The brogue and the burr, or whatever they’re called?
Thank you, Robert. As a native Torontonian (admittedly, now living in western Canada), I am tired of explaining to American friends why I don’t say “aboot” or “hoose.”
Interestingly, when I am in the US, nobody pegs me as Canadian based on my accent. Indeed, nobody seems to know where I am from unless I tell them. Up until I tell them, mostly I’ve been taken before for a Chicagoan; and on a few occasions, for a Nebraskan or Coloradan.
Sure, but the accent would be a dead giveaway in either country.
How about the people of Polynesia, Melanesia and Micronesia… All the countries, Fiji, Tahiti,
same language, same looks… similar culture.
There are exceptions …eg Hawaii is its own mix now, New Caledonia has the French, whereas Papua New Guinea are more pure Austronesian - not mixed with Asian.
There are important differences between Australian and New Zealand accents.
Or duffrences, as the case may be. 
Thailand and Laos to some extent. In fact, there are more ethnic Lao in northeastern Thailand than in all of Laos, and the Lao language, which is very similar to Thai, is the Thai dialect of much of the Northeast.
For the most part I agree, although many accents in say Co. Antrim and Co. Donegal can be difficult to tell apart from western Scottish accents. Not sure if they’re as plain as day to Scottish people but to Dublin ears it can be difficult.
There are similarities, especially amongst the accents of Ulster and parts of Scotland but a Co. Cork accent, for example, sounds quite unlike any Scottish accent I’ve encountered.
It’s easy: the ones who take offense but refrain from biting your head off when you confuse them for Argentinians are the Uruguayans. I’m in a similar situation at the regional level, so I can see why it can get a bit tired after a while. Thankfully the morons who insist that no way you can be from where you say, you have to be from where they say, are few and far between.
My first response to the OP was the individual Emirates of the UAE…
Thus. 
As for the OP, it’s hard to imagine 2 large countries more similar than the US and Canada. There are differences, sure, but pretty minor (if you exclude the political process, which isn’t really something people actually deal with on a regular basis).