Drupal - How can I deploy user interface translation?

Helper function to add custom translations:

/**
 * Helper to manually add a single translation string.
 *
 * @param string $source_string
 *   Source string.
 * @param string $langcode
 *   The langcode.
 * @param string $translated_string
 *   Translated string.
 */
function MYMODULE_add_translation($source_string, $langcode, $translated_string) {
  // 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);
    $string->setStorage($storage);
    $string->save();
  }
  // Create translation. If one already exists, it will be replaced.
  $translation = $storage->createTranslation([
    'lid' => $string->lid,
    'language' => $langcode,
    'translation' => $translated_string,
  ]);
  $translation->save();
}

Usage in an update hook:

/**
 * Add translations.
 */
function MYMODULE_update_8002() {
  MYMODULE_add_translation('Adjust', 'de', 'Anpassen');
}

You have to do this yourself. @rpayanm answered where you can do it manually, you will need to look at the code there, and for example implement drush commands that allow you to export and import it again. Automating that is then relatively easy, export, sync it to the other server, and import again.

Looks like a project that helps with that exists: Drush Language Commands.

You can put default translations in a module ,but those are only imported when you install that module. hook_update_N() is designed to only run once, so you would need to write a new function every time you want to do it (and you still need to export it first).

You can export the translated strings used by your site A (admin/config/regional/translate/export) and then import them on the other site (admin/config/regional/translate/import).