I don't think that cultures or languages would be negatively impacted merely because a heretofore artificial line has been erased...in fact, I think that people such as the Kurds, Roma populations, and the Royhinga people (spelling?) may even benefit from being more geographically and politically unconstrained.
As for who would govern, I think that it is high time for true democracy to reign instead of moneyed elites to buy their way to power. Local resources should probably be decided by local people, whilst globally needed resources by global committees for instance.
The UN is a toothless tiger that makes generalizations in deference to the idividual power elites that really decide what hapens on this planet.
I would like your sources on how "a one nation planet will be a mess and probably boring" please. Ditto for the overpopulation aside.

I think that you will find that, originally, we all WERE one society, and that the greed and bullying tactics of a minority altered the situation, resulting in the society that wehave today.

@Wageslave From what I can make out there were numerous stages in human evolution, many before homo sapiens and that there were many species (16 identified so far) in the genus homo, . There was no clearly defined step that made us homo sapiens instead it was a lot of small variations in different groups. The early days were family clusters, then tribes and so forth. I cannot see any point in time where there would have been a single global human society. The larger groups were formed by the more powerful taking over the lesser aggregating continually. Just look at the United States, The Commonwealth of Australia, The European Union, The Soviet Union. The earlier empires. Britain was tribal originally, Romans came and left, the Anglo Saxons came and were almost completely conquered by the Danes, when they got rid of the Danes they gradually merged the 7 Saxon kingdoms. After a few more invasions back and forth across the channel we ended up with the Empire upon which the sun never set.

Cava, your cherry-picked example of the Tragedy of the Commons, is a poor example. Firstly, this is merely an unsubstantiated opinion put forward as a 'hypothetical' reason, never observed in real life. Secondly, it would obviously fail as an isolated experiment within a competing paradigm! In a society where ALL resources were open to all, and where people merely took what they need for daily existance ( as opposed to building a store of wealth at everyone elses expense,) it would work just fine.
Particularly if Big Olga was keeping an eye on grazing times!

@Wageslave No it is a theory, which has some explanatory power, unlike your unlikely scenario.

I work at the basic community level, community gardens get raided continuously, immature produce even whole plants get taken, I visited a nearby town yesterday, the night before whole bunches of bananas were cut down and taken. Our community pantry opens at 10 am, people are there at 7-30 so as to have the first pick of what is there and argue that we have limits on how many of an item one person can have. Human behavior sucks.

@Cava "many examples have existed and still do exist where members of a community with access to a common resource co-operate or regulate to exploit those resources prudently without collapse.[4][5]"
From the source that YOU (Cava) quoted

@Wageslave The tragedy of the Common is a explanation of use of the Commons it can be positive or negative, however it is predominately negative in regards to your 'topic' as all of human history has taught us.

Ah, yes, war...SO acceptable!
We actually have the resources to accommodate 10 to 15 billion people on this planet, if only we were to organize things a little better...

@Wageslave Yes, it is. We are top of the food chain. Our other option is to take a lower place on the food chain. I would rather battle it out among ourselves, then have something bigger meaner and nastier pluck me for vittles. How long do you think it would be before we achieved population overload without war? I would hate who we would have to be to eliminate war. First, you couldn't have discourse. Couldn't have arguments. Any form of aggression. Maybe if we were cows...do cows fight? I honestly don't know....haha