Create translated strings manually in Drupal 8 config interface translation

manually_translate_d8_config.php

<?php

use \Drupal\locale\SourceString;

$source_string = "Source text string";
$translated_string = "translated string";
$langcode = "ar";

// Find existing source string.
  $storage = \Drupal::service('locale.storage');
  $string = $storage->findString(['source' => $source_string]);
  if (is_null($string)) {
    $string = new SourceString();
    $string
      ->setString($source_string)
      ->setStorage($storage)
      ->save();
  }
  // Create translation. If one already exists, it will be replaced.
  $translation = $storage->createTranslation([
    'lid' => $string->getId(),
    'language' => $langcode,
    'translation' => $translated_string,
  ])->save();