manslaughter
Yeah, it would take a legal expert in both "continental European" and "Anglo-Saxon" law... But at any rate Totschlag/doodslag is always willful. German Wikipedia's definitions of "Totschlag" for Germany, Austria, and Switzerland all specifically use the word "vorsätzlich" (= opzettelijk).
I think the solution might be the following. English wikipedia distinguishes between w:voluntary manslaughter and w:involuntary manslaughter. Thus it seems that the definition of "manslaughter" in Wiktionary would have to be changed because manslaughter can indeed be willful. And "voluntary manslaughter" is the German/Dutch Totschlag/doodslag.
"Involuntary manslaughter" would be something else. In Germany there is Körperverletzung mit Todesfolge ("assault resulting in death") and fahrlässige Tötung ("negligent homicide"). But this is another topic, that would get to specialized for myself.
But I would feel comfortable to change the definition of manslaughter to something like: A criminal act of killing a human being considered less culpable than murder (based on w:Manslaughter). And then I would add "voluntary" in brackets to the German/Dutch translations. That should be fine, right?