Code snippet that can be used to add active class to links language in Drupal 8.
/**
* Implements hook_preprocess_links__language_block().
*/
function THEMENAME_preprocess_links__language_block(&$variables) {
$language = \Drupal::languageManager()->getCurrentLanguage()->getId();
$variables['links'][$language]['text_attributes']['class'][] = "is-active";
}
