Ah, yeah, Marten Asmodom Vilijn.... The translator had to be creative as Tom Marvolo Riddle will spell "I AM LORD VOLDEMORT" when you mix the letters. Marten Asmodom Vilijn will form "MIJN NAAM IS VOLDEMORT" which literally means "My name is Voldemort".
"Hij die niet genoemd mag worden" is in literal translation (English grammar be damned) "He that not named may become", I guess it's an easy guess that's the Dutch version of "He who must not be named".
"Jeweetwel" is in Dutch a connection of three words. "Je weet wel". There is no real English word for "wel", but it's the opposite of "not", and "je weet" means "you know", so that's "You-know-who" in Dutch.
The name "Lord Voldemort" was never translated. Now the Dutch word for "Lord" may be "Heer", however it's a rarity to use that word in translations from English especially when referring to people of the British nationality.
Now the spelling "Sneep" may seem funny in English, but in Dutch you pronounce it exactly the same as "Snape" is pronounced in English, so technically the name didn't change, only the way it's pronounced.
You know what's funny in Dutch? The name of Hermione's S.P.E.W. or Society for the Promotion of Elfish Welfare. The Dutch name is S.H.I.T. (Stichting Huiself voor Inburgering en Tolerantie = Foundation House-Elf for Integration and Tolerance). And yeah the word formed by the abbreviation is just copied from English, and used in the same context.
