It is a well-known story that the original German manuscript of Darkness at Noon was presumed lost until relatively recently (2015), well after Koestler's death. The English translation by Daphne Hardy was thus used as a "master copy" and the source for translations to other languages (including German).
It is also generally acknowledged (at least nowadays) that this translation was deficient in many ways, as Hardy was not a professional translator and had to work in a hurry.(*)
Nevertheless, Koestler lived long enough after the publication of the novel. Presumably, given his fluency in English, he was perfectly able to correct the problems with the text himself after the WWII, especially considering immense popularity of the novel at the time. And/or he could fix the German "translation". Yet he didn't do it.
Why is that? Are there any accounts of his personal opinion on the English text? Was he actually satisfied with the text enough? Or he just didn't bother? (There are authors who don't tend to touch their past texts, just as there are those who constantly revise them. Perhaps knowing Koestler's inclinations in this regard might help to shed the light).
(*) When I read it, I felt that it was a rather poor translation from Russian at times: there were many calques and clichés from Russian phraseology. But this may have been intentional for the setting, and we know that Koestler knew enough Russian. I haven't read the new translation yet to compare how it feels in this regard.