Form copy is the survey's own interface text — everything that isn't a question. It's edited separately from your fields, in the survey's Copy section, and it's translated per locale exactly like field labels are.
Each of these can also vary based on a previous answer (a conditional value), the same conditional mechanism used elsewhere in the editor — most surveys never need this and can just fill in a single default value per locale.
Every one of these fields must have a value for every locale your domain supports — a blank translation blocks publishing the same way a missing field label does. See Form types for the full validation checklist.