How to update the language in a existing node entity?
You should be using the get()/set() methods (of the FieldableEntityInterface), sth like this:

// Get the current node's language.
$node-&gt;get('langcode')-&gt;value;
// Set the current node's language.
$node-&gt;set('langcode', 'de');
// Save the node to persist the change.
$node-&gt;save();


Hope this answers the question for you.