code: function() {
          return new Promise(function(resolve, reject) {
              try{
                  resolve(library.listSourceLanguagesLastModified());
              } catch (err) {
                  reject(err);
              }
          }).then(function(languages) {
              return listResourceContainers()
                  .then(function(results) {
                      // flatten modified_at
                      let local = {};
                      for(let info of results) {
                          try {
                              if(!local[info.language.slug]) local[info.language.slug] = -1;
                              let old = local[info.language.slug];
                              local[info.language.slug] = info.modified_at > old ? info.modified_at : old;
                          } catch (err) {
                              console.error(err);
                          }
                      }
                      return inferUpdates(local, languages);
                  });
          });
      }
docs: Returns a list of source languages that are eligible for updates.
  
  @returns {Promise.<Array>} An array of slugs