In the short story:
The Canterville Ghost, 1887
[Describing Mrs Otis]
Indeed, in many respects, she was quite English, and was an excellent example of the fact that we have really everything in common with America nowadays, except, of course, language.
The Court and Society Review in two parts Vol. IV, No. 138, February 23, 1887, pp. 183--186 [Mason, 12]. Vol. IV, No. 139, March 2, 1887, pp. 207--211 [Mason, 13]
Mason, 12, 13
Bibliography of Oscar Wilde
by Stuart Mason, 1914
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This is a chronology of attribution in print and makes no claim as to the origin of the thought beyond what appears self-evident.
The Canterville Ghost (see above).
Readers Digest, (November 1942): "England and America are two countries separated by the same language." [unsourced]
Repeated in Treasury of Humorous Quotations (Esar & Bentley), 1954. [also unsourced]
Repeated in The Oxford Dictionary of Quotations, 1999 p.521, under the heading Misquotations :
Saturday Evening Post, (3 June 1944): "It is a misfortune for Anglo-American friendship that the two countries are supposed to have a common language".
Quite Early One Morning, (1954)
A collection of essays and stories. In A Visit To America, p.146, European writers and scholars in America were, he said : "up against the barrier of a common language".
Repeated in a radio talk prepared shortly before his death and published after it in The Listener, (April 1954).
The Times, 26 January 1987; The European, 22 November 1991 : "Winston Churchill said our two countries were divided by a common language."
Special thanks to Nigel Rees, host of the BBC Radio quiz program "Quote ... Unquote", for use of his original research that forms a major part of this chronology.