How to quote a non-English quotation in dissertation?
The quotation is really more for fun than for substance, so you've got a lot of leeway, and even more so because this is in a Ph.D. thesis.  So really, it's up to you whether you translate it at all.  

That said, I would suggest approaching it based on the degree to which you like the prosody and meter of the quotation.  If the sound of the words themselves is a significant part of what makes you enjoy the quote, then by all means keep it in the original language, where it will be well appreciated by some readers and challenge the rest.  If the substance is more important than the sound, then it may be better to translate.

For example, I don't know much of German quotations, but if I were using a French quotation, then I would not translate "Ceci n'est pas une pipe," but I would definitely translate "Les grandes personnes ne comprennent jamais rien toutes seules, et c'est fatigant, pour les enfants, de toujours et toujours leur donner des explications."