It's actually just known by the romanization of the Japanese word. So for「コンニャク」in English, you can just use "konnyaku". Or if referring to a food with it instead of the plant itself: "konnyaku jelly".
This is a brief summary from the English wikipedia page on it: "Konjac, also known as konjak, konjaku, konnyaku potato, devil's tongue, voodoo lily, snake palm, or elephant yam, is a plant of the genus Amorphophallus. It is native to warm subtropical to tropical eastern Asia, from Japan and China south to Indonesia."