8 – How can I bulk update the language for all content?

Load the nodes entities and set its langcode column

For example:

$w_node = entity_load('node', $nid);
$w_node->set('langcode', $data_langcode);
$w_node->save();

To do it in bulk, i guesing that you should create a new action to be available in the defaults bluk operations for the admin/content view