"How do you say?"
Translation:Comment dites-vous ?
To mean "how do you say..." indeed you need to add something, which is the notion that you want the person to define or translate for you.
- Example: "comment dis-tu 'un cheval', en anglais ?" (how do you say 'a horse', in French?). In this case, you can also use "on" instead of "tu" (comment dit-on...)
However, if you hear someone telling you "comment dis-tu ?" it is just an invitation to repeat what you just said. Therefore, this is a direct address to you, and "on" is not suitable.
However, if you hear someone telling you "comment dis-tu ?" it is just an invitation to repeat what you just said.
Speaking for myself, as a British English speaker, I can't remember ever having used "How do you say?", nor heard any one else use it, to ask someone to repeat what they have said.
If I had misheard someone I would use "What did you say?", or possibly "What are/were you saying?" not "How do you say?"