As a Hungarian I have not heard of meanings of this word like "world of dead, this world and the next". It literally means twilight or semi-darkness (if such a word exists :D) but that does not mean that they would not use it in those meanings in some novels. The mood or the tone of the word is quite gloomy (for me at least) so I can imagine. :)

You probably need a Hungarian speaker to give you an exact definition of the word, but it sounds like you already have the correct idea, that the author was trying to convey. If it were simple to translate into English, the author probably would have done this! I think there is no direct translation because we don't historically have this concept or culture in Britain i.e. no connection between twilight and the dead, at least not to my knowledge. (I would actually imagine Mexico and Day of the Dead, when thinking about this topic.) I am not an expert on early pagan religion (the religion before Christianity was introduced to Britain), but Wikipedia says that they believed you go to the underworld when you die i.e. no resurrection. Then later in Christianity, Jesus was resurrected (while everyone else goes straight to heaven or hell), but again, no particular connection to twilight or coming back to earth. So, I would say that this idea just does not exist in the (original) English-speaking world, so we never developed a word for it. (Maybe Mexican Americans have a nice word for it today?) In some cases, where there was no English word developed (because there did not need to be), the foreign word gets permanently adopted into English e.g. "coup d'état" (from French) or "putsch" (German) are both used to describe an event for which we have no English word. Maybe one day, "Felhomaly" will become adopted into English!

Félhomály is semidarkness. When you can't see properly, but it's not that dark you can't see anything at all.