Two esteemed Foreign Policy bloggers have different takes on the spread of the English language and what that means for foreign diplomacy. First, Ali Wyne:
But for those who claim that the post-American world is a fait accompli, there is one big problem: The English language is winning hearts and minds faster than politics ever can. With the June 10 addition of "noob" (a pejorative description of a newcomer to a particular task or group) to its lexicon, English will boast one million words - twice as many as Cantonese, four times as many as Spanish, and 10 times as many as French. Half the world's people are projected to be speaking English by 2015. And so long as English is on track to become the world's unofficial language, the United States will likely be center stage. ...
Whether it's Latin during the first century or French in the 18th, great powers and global lingua francas tend to go together. So while the unipolar moment may be over, the growing influence of English will ensure that the United States doesn't fade into the sunset anytime soon.
Then Stephen Walt:
Americans sometimes view the dominant position of English as another component of America's "soft power," but that view is simplistic chauvinism. With English becoming a "universal" language, no single country will own it or be able to regulate its content. Instead, it will continue to evolve as most languages do, incorporating new words, spellings, and grammatical practices from an wide variety of sources. If they haven't started already, American xenophobes are going to start complaining soon about the corruption of "standard English" by all these foreign influences.
I score this one for Walt. The dominance of the English language in international settings is an effect of current (and past) power structures, not a cause, so arguing that the spread of the language will prolong American dominance is a major over-statement. After all, the United States was not even the country that initiated the spread of English to all parts of the world. That honor, of course, belongs to the British, on whose empire the sun never set. Without the reach of the British Empire to India and Hong Kong, would we expect so much penetration of the language into Chindia? Indeed, the economic center of the world for centuries was London, not New York, and much of the current use of English in Asia and Africa can be attributed to the Brits. However, despite the persistent prevalence of their language, the actual influence of the British has declined steadily over the past half-century.
This is not to say that the entrenchment of English as the global language does not convey some advantages for America, but it is very easy to over-state them. The French have long pushed the adoption of their language as the global standard, and have even had some successes -- French is one of the official languages of many international institutions, including the U.N. and the Olympics -- but these successes have done little to maintain France's influence in the world (or the adoption of the language). Many Americans are choosing to learn Spanish to better accommodate demographic shifts in the country (and thus improve economic opportunities), but it would be silly to argue that that signals a rise in the influence of Spain (or even Mexico) over American politics. It would be similarly silly to argue that America's influence would markedly fade if Esperanto was spoken in the U.N.
The concept of "soft power" is quite often useful for explaining the relations of states in the international system. But it is easy to make too much of something that isn't really there.
P.S. For more discussion, Walt recommends this roundtable hosted by the Freakonomics blog.