public function read( $identifier, ?string $sourceName = null, $defaultTranslation = false )
   {

      if ( null !== $sourceName && isset( $this->_sources[ $sourceName ] ) )
      {
         // read from specific source
         return $this->_sources[ $sourceName ]->read( $identifier, $defaultTranslation );
      }

      foreach ( $this->_sources as $source )
      {
         $result = $source->read( $identifier, static::USS );
         if ( static::USS !== $result )
         {
            return $result;
         }
      }

      return $defaultTranslation;

   }