What Multiple contributors means in Drupal Translations site?
See the Working with translation files offline documentation under the Contributing back to the community header:


  When you have finished, contribute the translation back to
  localize.drupal.org by importing the file on the translation team user
  interface. Translations will either be attributed to you or "Multiple
  contributors" depending on which option you choose. "Multiple
  contributors" is a special account you can use if multiple people
  worked on the translations being imported and singling out one
  contributor for attribution would not be correct.