Well, a few weeks ago I finished the translation assignment I mentioned above. The author did not enlighten me about that huge quote that made no sense to me so I did my best to translate it, and it actually sounds sensible now.
However, the author sent me a very arrogant and insulting email yesterday, saying my translation was "brimming with mistakes" and that I obviously didn't understand anything and had no idea about the basics of philosophy (I told him, by the way, before starting the assignment that my area of expertise was literature, not philosophy; the text covers both). I am shocked and have no idea how my translation can possibly be so utterly wrong! I don't think it is possible at all. Of course there may be one or two mistakes, but I did tell him from the start that I would incorporate any improvements if necessary. I am now waiting to get the list of my supposed errors so I can actually comment on them. It may take weeks till I receive it though.
I now regret having put so much time and effort into it all, even doing the research he was too lazy to do himself and reading up on philosphical concepts so I'd get them right. Obviously a waste of time. I am wondering whether he is simply trying to wriggle out of paying the rest of my fee.
EDIT: I found someone I believe was his translator before (may still be). I just generally asked her if she would recommend working with him and what it was like for her. I certainly didn't mention anything about my issues with him, of course.