It’s a Civil War song. The chorus of the union version includes lines like “the union forever” and “down with the traitor.” The lyrics of the confederate version include “our Dixie forever.” I don’t see how a song about a particular American war would serve as the anthem for a world government.
The Internationale.
…
What?
Neither does Gong Jin’ou.* I think that one would actually need less adaptation.
*Edit: my mistake, one reference to “heaven.” I still maintain it needs less localization, though. 
How about a song for the world government that isn’t US American? Or are you just assuming that any world government would be the US running the show?
Of course that’s what he’s assuming.
No, an American piece of music is just fine. What we need to avoid are such things as one country’s iconography and patriotism; references to one country’s revolution and success in war, and references to specific religions.
Thinking about the OP’s question, I’d suggest “Fanfare for the Common Man,” by (American–sorry!) composer Aaron Copland. It’s instrumental, so there are no lyrics to worry about or translate; and with a bit of title-tweaking (“Fanfare for the Common People”?), it could apply to everybody in the world.
Might as well use “Marching Through Georgia.” It’s a rousing tune, but the lyrics (once translated) would offend a whole other group of Georgians, too!
Please describe the type of government we are writing this song for, and the language it will be sung in.
The Olympic anthem – Aaron Copeland’s “Fanfare for the Common Man” – fits the bill as it has an appropriate title, there are no lyrics, and it’s already internationally known.
It’s known in this thread, too! 
Good point, Spoons. I like the idea of an instrumental song, too - no language worries there.
Now, where do we house this world government? The USA is out since you guys are about 30 years away from total collapse; I don’t like the idea of Switzerland since it’s too European with all that baggage (war here, war there, blah blah blah). How about New Zealand? They’re nice and neutral (as far as I know).
Along the same lines, how do we constitute it? A republican form, a Westminster parliament, something else totally, or a mixture of existing forms?
Is there a way to evaluate the various governmental forms used throughout the world and take the best of each?
To quote Twain, “Unlimited power is the ideal thing when it is in safe hands. The despotism of heaven is the one absolutely perfect government. An earthly despotism would be the absolutely perfect earthly government, if the conditions were the same, namely, the despot the perfectest individual of the human race, and his lease of life perpetual.”
But of course, as he notes, “But as a perishable perfect man must die, and leave his despotism in the hands of an imperfect successor, an earthly despotism is not merely a bad form of government, it is the worst form that is possible.”
I guess what I’m saying is that while it’s probably impossible to find the perfect leader…it may be possible to design and build one.
Copland’s (note spelling) Fanfare for the Common Man has never been associated with the Olympics as far as I know. The two main themes for the Olympics are the “Olympic Fanfare and Theme” by John Williams (1984) and “Bugler’s Dream” composed by Leo Arnaud (1958).
Curtis, would you mind stopping by and telling us what you want in a “World Anthem”?
The “serious” examples you’ve given us so far have been U.S.-centric and very Christian in nature. Are we to assume that this future government is based on some far right Christian idea?
This would be the most badass anthem for world domination:
Dimmu Borgir - Progenies of the Great Apocalypse
(linked is the “clean” video - there’s an NSFW version linked in the related videos that I think fits the spirit of the occasion better)
You know, now that I think of it, why does a world government need an anthem, anyway? Or a flag, for that matter? The whole purpose of those things is to dfferntiate one country from another, and if there is only one country…
If we as a world really need an anthem, I say write a new one.
I don’t think a world “nation” could be enough like any of the nations we have now to recycle an older song, most of which reference specific battles, nations, or religions, or was originally written for a specific language, and no matter what you choose, you’d end up alienating a very large portion of the world. Which is kind of the opposite of the point of an anthem, which is supposed to make the people who sing/hear it feel united.
My honest best guess is that if words are indeed desired for the anthem for a world government, they should commemorate how that government came to be. Until we know that, it’s hard to say what the anthem would be.
Of course some events might be too quotidian to make a stirring anthem; “The Consensus of UN Subcommittee 513, Promulgated through Plebiscite” probably won’t set too many hearts aflutter.
World government or no world government, “Night Train” should be played at all state occasions.