I have long thought the ideal of a single planetary government is an excellent thing for humanity to strive for. Long term, I think it's essential for the survival of the human race in order to maximize efficient utilization of Earth's resources as we expand into space. But I'm rather cynical about human nature. It will probably take some external existential threat of an extreme nature (such as the aliens of Independence Day) to force our squabbling species to finally set aside its differences and unite. I really don't see it happening in my lifetime, or my daughter's lifetime.
There have been some encouraging signs. The European Union is a grand experiment that shows how a planetary system of government might evolve. Although as we can already see, base human nature is starting to assert itself and I'm skeptical the experiment will survive.
Homogenization of human language and/or culture and loss of individual cultural identities
I would argue this is not a necessary consequence of a world government. The United States gets on very well as a collection of semi-autonomous states, with quite widely varying local customs & identities. Someone from the South is very different from a New Yorker or a resident of California, but they all identify as Americans. Of course, it gets harder to keep such a union together as the scale of it increases, and the variety of cultural differences grows, as the EU has proven. Fundamentally the concept remains the same, but the devil is in the details, as they say.
My current thoughts aren't far off from this. While on the one hand the potential benefits of 'things being pulled together' with the other having the 'human nature' and tendency for corruption/collusion/etc. I can't get myself to simply 'trust' in such bodies...at best I can trust individuals that (I think) I know personally vs the facade they present in media.
It's a good description of the US being semi-autonomous states. Personally I see that nicer medium with states having the ability to cater to more specific groups, which they somewhat already do...marijuana being a current case in point, even though federally there still is a blanket ban. It's interesting making that comparison with the EU, where even though a part of that group, I'd assume most people there identify as their nationality (French, English, German, etc.) versus being European.
But like you said, the devil is definitely always in the details.
That's the main challenge the EU has, getting people to identify as European. As the nation states of the EU existed long before the EU itself, people there naturally identify as their nationality, whereas in the US you identify as American first. The great advantage of the US is it was designed from the ground up for that integrated system of government, instead of having to change existing systems later on. Also there's no escape hatch in US law for states to leave the union. Having, say, California secede and become its own country would be unthinkable (no matter how much the west coast states hated to see Trump win).