16875 adding deleting parent and child relations not working in details pane

[``* `#16875` Adding/Removing parent relations not working in details pane``](https://community.openproject.org/work_packages/16875)
[``* `#16878` Removing child relations not working in details pane``](https://community.openproject.org/work_packages/16878)
this should be handled by I18n