Assume I have my site translated in three languages: en (default), fr, de.
Now assume there is a node with an english original and a german translation, but no french translation.
Node url aliases in my site are multi-language as well, so I have "en/about-us" for the english "About us", and "de/ueber-uns" for the german one.
In the german menu, I want a menu item pointing to the german translation. This works fine.
In the french menu, I want a menu item pointing to the english original, because there is no german version. This does not work, because all menu items to nodes in different languages will be automatically hidden.
How can I make a menu link that points to a node in the original language?