You could go for 'bastard bitch', as this would leave no doubt as to the person's gender, and keep the 'out-of-wedlock' insult intended. Just a suggestion, though. I personally feel that f***ing is not quite the right word here...
Oops! I've just seen Aztlan's post, it sounds like German
Thank you very much for your quick answers. I knew f**ing was not very appropiate, but it was the first thing that came into my mind. Nonetheless, I think I prefer Miguel Antonio's option, as Aztlan's seems very hard to understand for an ordinary audience (The quote was taken from a soap opera's dialogue).
Aztlan's suggestion is perfect, but SydLexia is right: it depends on whether the rest of the dialogue sounds ancient or modern.
It irritates me enormously to read a novel or watch a movie situated in ancient times but with a dialogue that sounds as if it came off the streets in 2012; however, if that is the way it is written, you'll have to follow through and translate it the same way.
The soap opera is about the Roman Empire and it takes place during the 2nd century BC. The dialogue is supposed to sound formal, so I am going for ancient. However, it shouldn't be extremely formal, nor extremely ancient as it is not a book, it is a show for an ordinary audience, as I have already said.
Here's an example of how I translated other insults:
¡Cerdo!¡Te arrancaré las tripas! ¡Te mataré! = You bastard!/You swine! I’ll pull your guts out! I will kill you!
I'd be reluctant to use the word bastard in this case. You don't tend to see it used as an epithet for a female (I mean, it would be perfectly correct to describe her as a bastard, but you wouldn't call her that as an insult). I'm not 100% convinced by "f**ing bitch", but I think it at least sounds more natural than "bastard bitch".