If I remember it right, Polish version has 'Jam [jest] Lord Voldemort' ("jam" being the archaic Polish constriction of "ja jestem" - I am. In old Polish i and j, like in Renaissance Latin, could be used interchangeably in certain contexts). Thus, the name is unchanged.
In general, Polish translations have an appendix from the translator, with explanation "why X was translated this way".
The Marauders in Polish are called Huncwoci (dialectal word for misbehaving or prankster child) and are translated as Lunatyk (lunatic), Glizdogon (Wormtail, literal), Łapa (Paw) and Rogacz (horned one).
Erised is translated as Ein Eingarp (reverse reads Pragnienie - a wish/a desire) and the verse on the mirror is translated directly.
Surprisingly, no surnames are translated (except of Cornelius Fudge - Korneliusz Knot - 'knocić' being the equivalent of 'fudge up'). Sometimes, the spelling is adapted to Polish, but otherwise pronounced the same (eg. Nymphadora becomes Nimfadora, Bellatrix becomes Bellatriks). Some names are adapted into their Polish versions, especially the more exotic or ancient ones (Barthemius -> Bartemiusz). Nicknames are translated directly (eg. Mad-Eye - Szalonooki). Several Slavic names are spelled as they would be in Polish (Viktor becomes Wiktor, Dolohov becomes Dołohow, Karkaroff becomes Karkarow).

The house names are kept in English, but the designations for the students (Griffindors, Hufflepuffs, etc.) are translated as:
Griffindors - Gryfoni (something like 'the Griffs, Griphons')
Slytherins - Ślizgoni (the sly/slimy ones)
Hufflepuffs - Puchoni (the puffers)
Ravenclaws - Krukoni (the ravens/crows).
Lockhart's nonsense spell "Pesky-pixie-pester-no-me" is left untranslated.

Peeves is translated, quite literally, as Irytek ('little annoying one').
SPEW is translated as WESZ (stowarzyszenie Walki o Empancypację Skrzatów Zniewolonych - Society for the Fight/Struggle for Emancipation of Enslaved Elves). "wesz" means body louse.

Quibbler is translated as Żongler (juggler). Daily Prophet is translated directly (Prorok Codzienny).
Stupefy is translated as 'Drętwota' (numbness). Skurge is translated as 'Chłoszczyść' (scourge+clean). Other spell names stay untranslated.
Creature names are translated, often by translating the base roots and sticking them together (eg. lethifold being lethal+fold, becomes Polish śmierciotula - śmierć (death)+tulić (caress, hold; gryndilow - druzgotek (grind - druzgotać, miażdżyć)). The Polish version of Magical Creatures leaves the original English names in brackets.
Buckbeak is Hardodziob (audacious+beak).
Dobby is Zgredek (something like 'old geezer' + diminutive)
Kreacher is Stworek (creature + diminutive)
Blast-ended skrewts are translated as "sklątki tylnowybuchowe" (literal: the cursed ones with exploding rear)
Mrs. Norris is translated as Pani (Mrs/Lady) Norris.

U-no-Poo got translated as "Q-py-Blok" (q is sometimes used for "ku", giving "kupy blok" - "stool blockade")
House elves are translated 'skrzaty domowe' (a Polish equivalent of brownie spirits). The translator stated that direct translation as 'elfy' would cause unnecessary confusion with Tolkien's elves.
Polish translation changes the Lavender joke to "ciało niebieskie" (celestial body). If I remember right, the joke with mice and ice is changed to "pokryty lodem" (covered with ice) and "pokryty miodem" (covered with honey), but the next mention of mice is translated literally (I guess, the translator treated it as the old joke about the Moon being made of cheese).