In Avengers: Age of Ultron, at one point, we see Seoul. However, the subtitle that introduces where we are, simply states 'Seoul, Korea'. In real life, Seoul is in South Korea, but in the film, there's no 'South'. Does this mean that in the Marvel Cinematic Universe, North and South Korea have reunified? Was this one of the impacts of Stark's 'privatized world peace' in Iron Man 2? Or is it normal to refer to Seoul as being simply in Korea?
There is nothing there saying that the term "Korea" must be a specific, single country. It could designate as well the whole korean peninsula; covering both the north and the south.
Something similar happens with the name "United States of America". When someone says that he is going to the United States or to America, he's not saying that the country has officially changed its name to its shorter version. Likewise, the term "Americans" doesn't include the Canadians, the Mexicans or the Brazilians.
Unless something else in the movie specifically states that the North and the South have been reunited in a single country, you cannot draw this conclusion from only a subtitle.
I think the issue here is that the OP has confused the term country and state.
In the vast majority of cases, a country is a state, a unit of government.
However exceptions are all over the place. A counter point would be, London, England. According to your rules that is not allowed. I MUST call it London, United Kingdom or London, British Isles (which I don't think you could point to on a map).
Another point would be New York, New York. Although most Americans would not realise it, US States are semi autonomous states of government. Therefore, it is just as valid as New York, USA.
The point is that Korea is a country and there are two states on it, which are at a state of war.
The question is what formula is Marvel using, [city], [state] (like you think it should) or [city], [country].
It should be pointed out that the official name in English of the Country commonly referred to as "South Korea" is "The Republic of Korea". Both North and South Korea consider themselves the "real" Korea.
While it's possible the omitted "South" descriptor was done with a specific intention, without further data, there's nothing which points to what that reason was, or what, if anything it means for the world of the Marvel Cinematic Universe (probably not anything).
This is how I see it as a Korean from South Korea.
South Korea is actually not a nation, it's just a part of the Korean peninsula. The nation in charge of the whole Korean peninsula is the "Republic of Korea"- to which people refer often as South Korea. Their (South Korean) constitution claims that the Republic of Korea to be the only legitimate government of Korean penninsula. (대한민국 헌법 제3조)
There is another nation called "Democratic People's Republic of Korea" established by the North Koreans, but South Koreans don't consider that as a valid independent nation. Their government is acknowledged to be completely illegal from a South Korean point of view. They don't follow the world's rule. They go against the United Nations and our constitution no matter what.
So that's why South Koreans just call their country Korea, and call themselves Koreans. Not South Korea or South Korean.
That can also be the reason South Korean people feel strange when they get a question like, "Are you from North or South Korea". They would never have thought that way (like how people from outside Korea see it) before getting that question.
Here are two analogies: (1) In most parts of the world, when you simply say New York, it will be understood that you are talking about New York City and not some farmland in upstate New York. (2) In most parts of the US, when you simply say Jersey, it will be understood that you are talking about the Garden State New Jersey and not some far-off island in the English Channel.
When one says Korea without specifying either North or South, it is understood that you are talking about South Korea. The reason is that the South is much more well-known. For one, it is vastly more affluent. It is also much more populous.
But most importantly, North Korea is completely isolated. Outside of North Korea (and perhaps South Korea and the northeast of China), you will meet a thousand ethnic Koreans without ever meeting a single North Korean.
You can survey a thousand world travellers and none will ever have travelled around North Korea (although some will have gone on one of those Disneyland-style tours of Pyongyang that ordinary foreigners can sign up for).
And thus, when you say Korea in the context of everyday life (and you are not a historian, political scientist, diplomat, or something along those lines), you will never be referring to North Korea.
Sokovia is also a fictional country, so the assumption that Marvel uses real geography and can be accounted for that is fallacious.
For information: Marvel "Stuttgart" is actually a copy made in Cleveland.. Let's only say that there is no „Bolzstaraße“ (The ending is "straße"), the main train station looks like a cosy bus station, our police cars has the old green-white designation and US sirens.