I've just been taking a look at the webpage, and even without having played it, I'm totally in love with it. I was rather skeptical at first (Nethack, by virtue of its complexity, should be difficult to translate, and even more difficult to translate well), but it seems that the authors managed.
And they did a great job, too! There's some information on various translations they chose on the Why and How page. Didn't you ever want to play a valkyrie and start at the rank of Küken? (Or a Grünschnabel if you're male?) Didn't you ever want to snack on Leckerlis (candy bars), or sit down and have a good Brotzeit (food ration)? Several of the scroll names are also quite clever, e.g. "Entrüstung" (destroy armor) or ""Entrückung" (teleportation).
But what's more, all the translations are just good. (And I like that they paid attention to little details; for example, the quest nemesis for valkyries is Surtr (Lord Surtur), and his lair is originally called "The Cave of Surtur", but they translated that as Surtshellir: the name of a cave in Iceland (which literally means "Surtr's cave", too).
The translators also took some other liberties, e.g. turning scrolls into Runen, and – and this is really rather neat – they also translated the commands and config options. I rather like the extended commands, particularly "kneifen" (quit) and "Lebenswandel" (conduct), and several of the config options are absolutely wonderful: "prayconfirm" becomes "Ehrfurcht" (awe), "autopickup" turns into "Raffgier" (the English "greed" doesn't do this word the least bit of justice: "raffen" evokes a mental images of somebody reaching out with both arms, pulling a stack of items towards himself and guarding it jealously) and "safe_pet" turns into "tierlieb".
Furthermore, "fruit" becomes "Leckerchen" (not related to the aforementioned "Leckerli"), "pickup_types" becomes "Raffgut" (tieing into the aforementioned "Raffgier"), and – last but most certainly not least – "scores" becomes "Ruhm_und_Ehre": a standing phrase that would literally translate to "honor and glory", but idiomatically to just "fame", another translation that fails to capture just how neat the German term sounds.
I'll definitely have to download this now and give it a whirl.
EDIT: I did, and I'm sad to say it still has various issues, from untranslated bits ("Du unterbrichst searching") to superfluous words ("Willst du wirklich den den Priester von Crom angreifen? or "Willst du wirklich den die das Izchack angreifen?") to sloppy bits ("So wie du die Rune liest, verschwindet die Schriftrolle." — what is it now, a scroll or a rune?) to wonky grammar ("Der Stab der Erleuchtung glüht, dann verblaßt.") and more ("Des Gnomen Dolche zerschell von der Wucht deines Schlages!" — this one works OK if the monster is only wielding a single weapon, at least). There's even outright mistranslations (e.g. "Du kommst dir gefährlicher vor!" for "You feel you could be more dangerous!"), and things like precious gem identification after death also need work ("ein eine ein nichtverflucht... ein eine ein Amethyst"). Looks like it needs more love.
On the plus side, I noticed some more amusing tidbits; for instance, some of the scroll labels got translated, so "ELBIB YLOH" became "TFIRHCS EGILIEH" (which makes sense once you think about it).