QUESTION
What is the typical Spanish translation of the English idiom "to talk behind someone's back" (as in saying something bad about another person to others instead of to them directly)?
ANSWER
One typical translation is
Hablar a espaldas (de alguien).
The DRAE also suggests:
TweetHablar por las espaldas.
- loc. verb. Decir contra alguien, en su ausencia, lo que no se le dirĂa cara a cara.