Odd behavior with menu translations

[Zasa-san]

Menu items can be missing in translations collection (only checked the UI, need to confirm what happens in the DB) under some conditions.

• In a collection with at least two languages create a menu with only root items (no groups)
• Save
• Create a group and drag an existing item into the group
• Save again
• Check the system translations for Menu, the root item moved into the group is not present.

@txau detected a case where in the Menu collection you can find entries that are no longer present in the menu itself, this could be related to the issue described here.