Missing translation string for language names since 8.3.x

Moving this to the potx queue, as this appears to still be an issue.

In #2776975: March 3, 2017: Convert core to array syntax coding standards for Drupal 8.3.x RC phase → the syntax in LanguageManager.php was updated from

    return array(
      'af' => array('Afrikaans', 'Afrikaans'),
..,
      'lo' => array('Lao', 'ພາສາລາວ'),
...

to

    return [
      'af' => ['Afrikaans', 'Afrikaans'],
...
      'lo' => ['Lao', 'ພາສາລາວ'],
...

Since then, language names included in core have not been translatable on l.d.o:

https://localize.drupal.org/translate/languages/de/translate?project=dru...
https://localize.drupal.org/translate/languages/de/translate?project=dru...

Whether the translated names of these languages are actually used or not, I'm not sure, but this was originally reported in the core queue and is a regression, so moving it over here.