The Wikipedia page on Darkness at Noon claims that its predecessor The Gladiators was first published in Hungarian, while the Wikipedia page on The Gladiators itself claims it was first written in German. I've found other unreliable sources on the internet making both claims, perhaps taken from Wikipedia. Which is true?
It was written in German. In a postscript to the 1969 Danube edition of The Gladiators, Koestler mentions that The Gladiators was among the last of his writings in German, just before his switch to writing predominantly in English:
I wrote first in Hungarian, then in German; and from 1940 onwards, when I settled in this country, in English. The Gladiators belongs to the end of the German period. It was translated by Edith Simon, then a young Art student, who has subsequently become one of the most imaginative practitioners of the art of the historical novel. (p. 319, emphasis added)
In his article "How Der Sklavenkrieg became The Gladiators: Reflections on Edith Simon’s translation of Arthur Koestler’s novel", Henry Innes MacAdam describes and quotes from his correspondence with Simon's family members. While Simon's sister Inge remembers Edith's working with a German typescript, she is unable to say what happened to that typescript after Edith completed the translation. MacAdam claims:
There is circumstantial evidence that the document seen by Wessel in March 2016 is actually the “ribbon copy” which Edith Simon translated and later returned to Koestler. The hand-written note Revidierte Fassung: Original that appears on the first page of the Moscow MS points to such a conclusion. That would explain why a back-translation into German had to be done a decade later: neither Simon, nor Jonathan Cape, had kept the original typescript on file in London. (p. 48)
Whether or not this is correct, it is certain that Simon's translation was from a German typescript supplied by Koestler.
As Howard Gaskill points out in his answer to a different question, Koestler maintained several typescripts of his novels in progress, to guard against the danger of losing his work to confiscation by the pro-Nazi authorities. All original German typescripts of both The Gladiators and Darkness at Noon were lost during World War II. At least one typescript for each was eventually recovered. A typescript of Der Sklavenkrieg was discovered in the KGB archives, no less, in the 1990s by one Michael Scammell. Koestler had left it behind during his chaotic flight from Paris to London in 1940. Either the Nazis or the Vichy regime confiscated this script, which then found its way to Berlin, and subsequently to Moscow at the end of the war. A typescript of Sonnenfinsternis was found in 2015 among the papers of Emil Oprecht, the founder of the Swiss publishing house Europa, by one Matthias Wessel. Koestler had sent it to his publishers in Zurich before leaving France.
The Gladiators was published in English in 1939. In his autobiography The Invisible Writing, Koestler calls this English translation "my first novel to appear in print" (p. 393). He also says:
The novel was first published in England in 1939, in an excellent translation by Edith Simon. Then the war broke out, the German manuscript was lost during my flight from France, and the German edition which appeared after the war had to be re-translated from the English translation. A similar misfortune befell the next novel, Darkness at Noon. (p. 267)
There is no mention anywhere in Koestler's autobiography or in MacAdam's article of an original Hungarian version of this novel. Like Darkness at Noon, The Gladiators was written in German, translated and first published in English, back-translated from that English edition into German again, and only recently republished from a German typescript.
Flood, Alison. After 80 years, Darkness at Noon's original text is finally translated. The Guardian 24 September 2019. Retrieved February 25, 2023.
Koestler, Arthur. The Gladiators: With a new Postscript by the Author. London: Hutchinson, 1965.
Koestler, Arthur. The Invisible Writing: An Autobiography. 1954. Boston: The Beacon Press, 1955. Retrieved from archive.org February 25, 2023.
MacAdam, Henry Innes. "How Der Sklavenkrieg became The Gladiators: Reflections on Edith Simon’s translation of Arthur Koestler’s novel". International Journal of English Studies vol 17 no 1 (2017), pp. 37–59. doi: 10.6018/ijes/2017/1/258981. Retrieved from revistas.um.es February 25, 2023.