How would you translate 'what have you come as?' I've never heard this expression
You probably won't come across too many instances of "What have you come as?" in AmE.
Dog Lover's "What are you going to come as?" is plausible, but unnecessarily complex - "What are you going as?" would serve.
"What have you come as?" is more commonly expressed as, "What/Who are you supposed to be?".
It's more common in the past tense, third person: "They came as the ghosts of George and Martha Washington." (US)