Adding an existing node as a translation I have many articles in original (English) language and they're all translated to 3 languages - however, the translations were created as new nodes (and not by clicking "Add translation" in the Translation tab). 

Is it possible to somehow relate the translations with the original English article or do I have to manually redo it all by clicking "Add translation" in the Translate tab of each node and adding the translations?
You can do this in code by reading the field values from the three translated nodes and add them as translation to the english node:

$node_en-&gt;addTranslation($langcode1, $node1-&gt;toArray());
$node_en-&gt;addTranslation($langcode2, $node2-&gt;toArray());
$node_en-&gt;addTranslation($langcode3, $node3-&gt;toArray());
$node_en-&gt;save();


This is for demonstration only, you can loop through the languages and get the langcode from the node.

The problem is that at the moment the translated nodes and the english node have no connection in the database. If you have some data in the nodes that do match, you can do a entity query and loop through the results.