How to add language CSS class to Body in Drupal 8

It is easy as that, just open you .theme file in your theme and add the code below.

/**
 * Implements hook_preprocess_HOOK() for HTML document templates.
 */
function YOURTHEME_preprocess_html(&$variables) {

  $variables['attributes']['class'][] = 'language_' . \Drupal::languageManager()->getCurrentLanguage()->getId();

}

Now just clear the cache and new language class should appear in your body classes array. It should be something like language_XX