The information in the German web on this is contradictory, so I'm not sure. While two articles mention that she translated them herself, another source says that the publisher decided on the names before she began working.
The nephew's names have been translated quite diversified in general:
English Original: Huey, Dewey, and Louie
Chinese: 路儿,杜儿 和 辉儿
Danish: Rip, Rap og Rup
Finnish: Tupu, Hupu ja Lupu
French: Riri, Fifi et Loulou
Italian: Qui, Quo, e Qua
Dutch: Kwik, Kwek, en Kwak
Swedish: Knatte, Fnatte och Tjatte
Spanish: Hugo, Paco y Luis / Hugo, Diego y Luis / Juanito, Jorgito y Jaimito (from different translations)
Turkish: Cin, Can ve Cem