I blogged about Graham Greene's novel Stamboul Train once before, in Esperanto. This time here is a direct quote:
‘Budapest.’ Dr.
Czinner ceased writing for a little more than a minute. That small
pause was the tribute he paid to the city in which his father had
been born. His father had left Hungary when a young man and settled
in Dalmatia; in Hungary he had been a peasant, toiling on another
man’s land; in Split and eventually in Belgrade he had been a
shoemaker working for himself; and yet the previous more servile
existence, the inheritance of a Hungarian peasant’s blood,
represented to Dr. Czinner the breath of a larger culture blowing
down the dark stinking Balkan alleys. It was as if an Athenian slave,
become a freed man in barbarian lands, regretted a little the
statuary, the poetry, the philosophy of a culture in which he had had
no share. The station began to float away from him; names slipped by
in a language which his father had never taught him. ‘Restoracioj’,
Pôsto’, ‘Informoj’. A poster flapped close to
the carriage window: ‘Teatnoj Kaj Amuzejoj’, and mechanically he
noted the unfamiliar names, the entertainments which would be just
opening as the train arrived at Belgrade, the Opera, the Royal
Orfeum, the Tabarin, and the Jardin de Paris.
Graham Greene, Stamboul Train (Penguin paperback, p. 136)

Greene substitutes Esperanto for Hungarian here, making two mistakes in the process. I do not know why he chose to utilize Esperanto here, and why Hungarian in particular. Could it be a coincidence that since Budapest was the world cultural capital of Esperanto between the world wars, Greene identified Esperanto with Hungarian?
On Greene's concerns about language, here again is a paper with an updated link:
Going Especially Careful in The Third Man: A Linguistic Exploration by David Crystal.
Paper given to the Graham Greene Festival, Berkhamsted, September 2009.
Crystal finds the danger-signals in Greene's fiction often connected with
language. Artificial languages are markers of especially ominous
developments. There is nothing particularly ominous in the quote above as far as I can tell, but see an earlier post on Greene for a list of references in other novels. There are separate treatments of Greene's postulated artificial language 'Entrenationo' in his novel
The Confidential Agent.