add CSS class to body when masquerading

In an attempt to help the maintainers, I am going through some of the Drupal 7 issues, since it's EOL ... So thanks for making this request @bassplaya, is it still relevant for 2.x in Drupal 11?

Thanks for a fast feedback, let's close it then.

I added the current language as a custom CSS class to my multilingual site arst.dk in Drupal 10 (Drupal's multilanguage support is spectacular) via Twig templates. First define the variable in a custom module:

function mymodule_preprocess(&$variables) {
  $current_language = \Drupal::languageManager()->getCurrentLanguage()->getId();
  switch ($current_language) {
    case $current_language == 'it':
      break;
    case $current_language == 'fr':
      break;
    default:
      $current_language = 'da';
    }
  $variables['page_language'] = 'lang-' . $current_language;
}

... then use in custom theme file html.html.twig like this, adding for example lang-fr:

{%
  set body_classes = [
    logged_in ? 'user-logged-in',
    page_language ? page_language,
  ]
%}

Good luck with the English teaching!