Perhaps I've had too many high school language classes. It looks to me like textbook Dictionary Idiocy. Like the guy in my (supposedly advanced) French class who wrote "Les gens que je pends dehors avec" for "the people I hang out with," or others who look up "fly" and use "mouche" for "fly" as in airplanes, or use "confiture" for "jam" as in traffic, because it's what their dictionaries tell them and they don't think about it too hard.

It's also a book that takes itself a little bit too seriously for shoujo manga Cute. I could be entirely wrong there, though. The author admits that she knows nothing at all about Japanese as a language--perhaps not even enough to link "shisutaa" with "sister," it doesn't stand out so much as a non-Japanese word if you can't read the katakana...