It seems the strings from the screenshots (among others) are coming from this file:
~/plank/docklets/Trash/TrashDockItem.vala
But I do not know how to modify a program in Vala to enable the internationalization.
Moreover, the issue seems to be the same for all the .vala files in the ~/plank/docklets folder.
Attached is an other screenshot for the Clock docklet for instance.
That is why I think the bug title should be:
"Docklets contextual menus are not translatable" instead of "Trash desklet contextual menu is not translated"
Bug confirmed, attached are some screenshots of the untranslated strings.