When working with Panels in Drupal 7 it is possible to add an existing node to a panel. The relevant part of the UI looks like this:
Its help text says you can just type the node ID straight away, or use URL arguments to identify them. This works as long as the website isn't multilingual. As soon as it's multilingual though the ID entered in this field doesn't have to be node ID but the translation node ID, or tnid, of the node that has to be displayed in the panel.
The help-text in that case is wrong.
You'd expect you'd be able to add the node ID and Drupal would get the tnid of the node when it needs to display it, but it doesn't. It just assumes it is the tnid and shows the node in the correct language with that tnid.