Timex and tomhudson have been talking ad nauseum about the whole "national language" movement in the United States.
I think it's a goos idea to have one, largely because it means that the government can restrict having to print ballots and such to the specified language(s). That's a money saver, right there.
English should be a no-brainer in the United States. Regardless of what the locals speak, the traffic signs here are *all* in English. There isn't one in any other language that might be construed as a local "favorite".
In the end, what is the point of a language? Communication. A common national language means that the nation's citizens will be able to understand each other. (Agreeing with each other is something else altogether, as one may have noticed from the way the politicos are always at each others' throats.) Specifying a national language should make things easier for the citizens to understand what their government is doing, too.
There you have it: my opinion.