Issue #3292391 by sanduhrs: Implement migration for Groups, Nodes, Users:...

    $plural_formula = \Drupal::service('l10n_pconfig.drupalorg_plural_formula');

    $formulae = $plural_formula->getDrupalPluralFormulae();

    if (isset($formulae[$row->getSourceProperty('language')])) {

      $row->setSourceProperty('formula', $formulae[$row->getSourceProperty('language')]);

      $value = $formulae[$row->getSourceProperty('language')];

      if ($value == 0) {

        $value = 'nplurals=1; plural=0;';

      }

      elseif ($value === '($n!=1)') {

        $value = 'nplurals=2; plural=(n!=1);';

      }

      $row->setSourceProperty('formula', $value);

    }

    return parent::prepareRow($row);

  }