Well, in a nutshell: In Japanese you have the possibility to add implications to your text by 'customizing' the reading of certrain words/expressions. In this case, we have the Japanese word for "paradise" (= 楽園), which usually reads rakuen. But Sakaki wants her title not to be read the 'usual way' but as paradisu (パラディス). If it were the English word "paradise" it would have to be read as paradaisu (パラダイス).why should it be read as a german title? sry i don't know much about the japanese language, but this sounds somehow weird. i mean kubo does a lot of language stuff, but that is something different i guess.
It may be a little hard to understand if you don't know that much about Japanese, so maybe it's easier to turn to phonetic transcription: the sounds after the "parad-" differ. One is 'i' whereas the latter is 'aɪ'.
But in the end it's really not that big a deal to begin with. I just thought I might respond to FanOfAniManga's interjection.
And don't wonder about mangaka who're playing around with readings and foreign/fantasy words. They just love that stuff because it sounds 'cooler' that way...
(Kubo is a primary example, of course. But you can find them in virtually every other series from Weekly Shounen JUMP.)/sorry for the little offtopic digression!
