code: function (data, oldContext, newContext) {
              // this function is called only if the language has changed
              if (newContext.language === undefined) {
                  return _.keys(data[oldContext.language]);
              }
              if (oldContext.language === undefined) {
                  return _.keys(data[newContext.language]);
              }
  
              return _.union(_.keys(data[newContext.language]), _.keys(data[oldContext.language]));
          }
docs: Determines if there are preference IDs that could change as a result
  of the context change. This implementation considers only changes in
  language.
  
  @param {Object} data Data in the Scope
  @param {{language: string}} oldContext Old context
  @param {{language: string}} newContext New context
  @return {Array.<string>|undefined} list of preference IDs that could have changed