Once you submit the file, you will get a wikitext result back. The file will contain <!-- CUT HERE --> markers. You are expected to copy each section until next such marker (or until end of the file) and place each them to a separate wikipage (such as Survey1, Survey2). This is because an issue in Translate extensions (which I discovered while doing this) which limits how big translatable pages can be. You must also mark each page separately for translation (required translation admin rights). After that you should go to Special:AggregateGroups and place all those pages in a single group, so translators can translate the whole survey together.