Warforged are an incredibly dumb idea that turned out to be amazingly great.
I wouldn't use them anywhere but in an Eberron game, but there they feel perfectly in place.
The word most probably is. The current word for corpse in modern standard german is "Leiche". (But it's pronounced very differently than it looks to English speakers.)
But the D&D monster is almost taken straight from a mythical sorcerer from Russia. Koschei the Deathless has been portrayed in images as an incredibly old man who looks almost like a walking skeleton, and he is some kind of sorcerer, and also immortal because he has a phylactery exactly as the D&D monster.
Koschei was also described in Frazer's The Golden Bough, which is still one of the ultimate introductory works on mythology. And I think Gygax even included it in some "Further Reading" lists in the back of some of his D&D books.