Select translation: where? In Drupal 7 (i18n) there is a possibility to select an existing node as a translation of another node.

In Drupal 8, is there also that possibility?
No, that was the old form of multilangual sites. This was removed in Drupal 8 in favor of entity translations.

Now every field had its own language instead of the node. This means the common properties of the node (nid, author, etc) can remain untranslated, while the values of the individual fields can be translated. (it prevents duplicate content)

So there is no way to select one node as a translation of another.