Discussion on: Drupal 8.x - What I've Learned

[Norah Shannon]

It appears you should be able to change the default interface language by:

1. Enabling Multilingual modules
2. Add the language under config > regional and language > languages
3. Select as default

Also, you can change the language code for entities by either:

• programatically looping through your entities, and setting the langcode value. Ex:

$node = Node::load($nid);
$node->get('langcode')->value = 'da';
$node->save();

• running SQL commands through all the tables where langcode is defined and changing the value

Sources:

• Change Interface Language
• Change entity langcode programmatically
• Change langcode stored in db

That being said, you would still need to change the actual content to the new language, as well (manually).