Comment 5 for bug 52537

Another vote for this one since Hardy has also not fixed this bug.
Current workaround for Wikipedia search plugin in German:
Browse to the German Wikipedia page, search for something and and look for the search bar area in Firefox. There you should see that the drop-down arrow/buttom is highlighted in orange. Clicking on it will give you a new proposed entry "Wikipedia (de) hinzufügen" (meaning to add Wikipedia search in German).