I'm trying to remember the name of an English-language novel that I read in the late 1990s. It was probably published in the 1990s or the 1980s.
The backstory was complicated (and seemed like it might have made for a better plot than the book's actual main plot). There had been an extensive civilization in orbit around the Earth, but it had been wrecked by some kind of space monster. (I envisioned the monster as insectoid, but I don't know if that was from the book or something I made up.) After the collapse of the sky civilization, there was a lot of violence on the Earth's surface, with "holyfolk" killing people who had too much technology.
The protagonists were two teens, a boy and a girl. The boy was the son of whoever had ultimately killed the space monster. The girl was the daughter of a man who claimed to be the last heir of the dynasty that had ruled the sky civilization. He was reduced to leading a band of nomadic raiders around Africa. When he died, the other nomads killed his second-in-command and abandoned the daughter.
Somehow, the two protagonists end up on a space station operated by a powerful interstellar civilization. Three evil aliens (of three different types; one of them resembles a blood-drinking lamia) arrive with them. While they are there, they all undergo a challenge to prove their worth to the interstellar powers. The humans are placed on a team with a lump-like alien, who goes catatonic for a while, after it sees how unqualified its teammates are. Yet the team somehow finishes the challenge.
I don't remember what their reward was, but the story somehow ended on a positive note.