Recognize the name Lamont Cranston?
The Shadow!
334 Shadow novels were written between 1931 and 1967, by several authors all using the pseudonym Maxwell Grant. Most of the stories were written by Walter Brown Gibson. His story Crime Under Cover appeared on 1 June 1941, and is unique in that Esperanto dialog is sprinkled throughout. The story also contains a brief (and slightly imperfect) description of Esperanto in an endnote.
Crime Under Cover can be found online here and here.Men were seating themselves at the next table, and Jerry could overhear their conversation. It wasn't in English, but Jerry had traveled around enough to know smatterings of other languages to the point of identifying them.
This happened to be one that he couldn't class, though it had snatches of familiarity. (Note: Unknown to Jerry Croft at this time, the language being spoken in the cafe is Esperanto. Jerry will learn the meaning of this restaurant conversation later in the story. Throughout the story; if the text itself does not make the meaning of the Esperanto clear, when it is spoken, a footnote will give the translation to the reader. For further information about Esperanto, please turn to the end of this story.) At least, the men spoke plainly, so that their pronunciation was recognizable.
"Mi sekvis lin el la stacio," said one. "Li estas la viro."
"Bonega," spoke another. "Ni observos lin."
Partly from memory, partly by reference to his notebook, Jerry was repeating words that he had heard Malga's men use in their conversation.
Harry nodded when he heard the words, and stated:
"Esperanto."
"Esperanto!" exclaimed Jerry. "I've heard of it. An international language, isn't it?"
"Precisely," replied Harry. "Perfectly suited to Kurd Malga and his tribe of spies. They are international themselves, hence they have become educated in a common tongue. Besides concealing their individual nationalities, it gives them the extra advantage of having a means of communication which the ordinary listener, no matter what his race, cannot fully fathom."
"'Mi sekvis lin el la stacio,'" repeated Jerry, lamely. "'Li estas la viro.'"
"'I followed him from the station,'" translated Harry, who knew his Esperanto. "'He is the man.'" Then, after a momentary pause: "They were referring to you, Croft."
"'Bonega.'" Jerry hesitated. Then: "'Ni observos lin.'"
"'Excellent. We shall watch him.'"
"The waiter said: 'Oni audos vin,'" recalled Jerry. "I remember the words 'komprenas' and 'malbona' —"
"'People will hear you,'" interpreted Harry. "Probably the rest was: 'If anyone understands, it would be bad.' What else did they say?"
Jerry studied the notebook. This time, he was positive that he had struck the crux of the thing.