On screen the actors speak English in their natural accents. However, all speech that is heard through artificial means is in Russian. All dialogue is in Russian if it is heard through the radio, emergency telephone call recordings, Soviet television news, and announcements made through public address systems. Because of this convention the makers were able to re-broadcast the actual recordings of fire operations dispatchers retrieved from the archives.
People refer to each other in the form "Comrade-surname," which is inappropriate among colleagues. Dyatlov's subordinates would have called him "Dyatlov" among themselves and "Anatoly Stepanovich" (his first name and patronymic) when addressing him directly, rather than "Comrade Dyatlov." However, it is likely the writers decided against using the correct forms of address to avoid confusion with non-Russian viewers, who might think, for example, that Stepanovich is Dyatlov's surname.
English, Russian, Ukrainian