The editors can choose if they want their admin interface in either language by logging in on the user page for any available language.
My problem now is that on the node create/update page, Drupal seems to use the global
$language to determine what options to retrieve for the Parent item under Menu settings, rather then using the current node's language.
So if an editor is editing a node in the default language but on the english interface, only english menu items will be in the select list mentioned above. When saving the node, the Parent item will be reset to the base of the menu and the previous menu structure will be lost.
What I want is a way of altering the chosen language for when that option list is being built. I suppose one way would be to use
hook_form_alter and rebuild the options for that field, but that doesn't feel like the right way of doing it.
Is there another way of solving this issue?
It doesn't necessarily need to work perfectly. I guess one way would be to have one language neutral menu and then localize the menu items instead. That would bloat the main menu though, especially when there are multiple languages.
Yet another hack would be to do as @reptilex suggested in his answer and then overwrite the admin translations with one language. I'd prefer not to though.