I do not think this is either possible, necessary, or even desirable.
The U.N. is an early attempt in that direction, but, even after more than fifty years, it is wrought with irreconcilable difficulties. Even the (in principle apolitical) UNESCO (which I once had the privilege of serving) has not been as successful in working peacefully.
Humanity is rich as a result of its variety: in cultures and languages, religions and customs, ideals and aspirations, local pride and ethnic pride. These are best nourished in separate units.
What is important is that these be shared in respectful and mutually enriching ways, instead of one group or all groups dominating together.
Valuable as diversity is, each nation should have only one primary language and framework of laws in the public arena, even while encouraging and opening the minds and hearts of the people to different languages and systems.
At the international level mutual respect and trust, as well as absence of intrusions, are far more important than trying to have one hodge-podge of an artificially unified government. The EU, UAR, OAS, and other regional governmental associations have revealed the difficulty in compulsory unification of people with different cultures, languages, values and uneven economic levels.
More seriously, it is difficult for minorities (in race, religion, language) to feel fully comfortable in any nation, living as minorities, no matter how much equality there is on paper and how much the majority proclaims equality in theory and in practice.