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.