              user = authenticate(username=username, password=password)
  
              if user:
                  if not user.is_active:
                      msg = _("User account is disabled.")
                      raise exceptions.ValidationError(msg)
              else:
                  msg = _("Unable to log in with provided credentials.")
                  raise exceptions.ValidationError(msg)
          else:
 
</s> remove                 msg = _("Unable to log in with provided credentials.")
 </s> add                 msg = _('Unable to log in with provided credentials.') </s> remove             msg = _("Must include \"username\" and \"password\"")
 </s> add             msg = _('Must include "username" and "password".') </s> remove             raise exceptions.AuthenticationFailed(_("Invalid username/password."))
 </s> add             raise exceptions.AuthenticationFailed(_('Invalid username/password.')) </s> remove             msg = _("Invalid basic header. No credentials provided.")
 </s> add             msg = _('Invalid basic header. No credentials provided.') </s> remove             msg = _("Invalid token header. No credentials provided.")
 </s> add             msg = _('Invalid token header. No credentials provided.')
                    msg = _('User account is disabled.')
              user = authenticate(username=username, password=password)
  
              if user:
                  if not user.is_active:
                    msg = _('User account is disabled.')
                      raise exceptions.ValidationError(msg)
              else:
                  msg = _("Unable to log in with provided credentials.")
                  raise exceptions.ValidationError(msg)
          else:
keep keep keep keep replace keep keep keep keep keep
 prefer single quotes in source and double quotes in user visible strings; add some missing full stops to user visible strings 
rest_framework/authtoken/serializers.py