Comment schema and object now uses langcode instead of language

Comment object language codes used 'language' in Drupal 7, and as part of a standardization effort to change to 'langcode' where the data is not a language object, the comment object was changed as well.

Drupal 7:

// Figure out the comment language.
$comment = comment_load($cid);
return $comment->language;

Drupal 8:

// Figure out the comment language.
$comment = comment_load($cid);
return $comment->langcode;