The values of Language::DIRECTION_RTL and Language::DIRECTION_LTR (used to be LANGUAGE_RTL and LANGUAGE_LTR) are now 'rtl' and 'ltr'

In Drupal 7 (and in Drupal 8 previous to this change) the language direction constants had the following values:

// Drupal 7
LANGUAGE_LTR === 0;
LANGUAGE_RTL === 1;

// Drupal 8 before this change:
use Drupal\Core\Language\Language;

Language::DIRECTION_LTR === 0;
Language::DIRECTION_RTL === 1;

These values were not very intuitive, especially as they showed up in configuration exports.

Thus, these constants have now been changed to be self-explanatory strings 'ltr and 'rtl.

// Drupal 8 after this change:
use Drupal\Core\Language\Language;

Language::DIRECTION_LTR === 'ltr';
Language::DIRECTION_RTL === 'rtl';

This also means that these constants can be used directly for the lang HTML attribute:

// Drupal 7:
$render_array['#attributes'] = array(
  'dir' => $language->dir == LANGUAGE_RTL ? 'rtl' : 'ltr',
);

// Drupal 8:
$render_array['#attributes'] = array(
  'dir' => $language->getDirection(),
);

Note that this change affects active configuration.