storageKey = function(key) {
    if (!key) {
      if ($storagePrefix) {
        return $storagePrefix + $storageKey;
      }
      return $storageKey;
    }
    $storageKey = key;
  }
@ngdoc function
@name pascalprecht.translate.$translateProvider#storageKey
@methodOf pascalprecht.translate.$translateProvider

@description
Tells the module which key must represent the choosed language by a user in the storage.

@param {string} key A key for the storage.