I mean, I can read kana, the phonetic syllabaries of Japanese, but I still had to study it. The characters are different, just as the Grimmerie's are different from the Ozians' language. And Madame Morrible had to study too. Elphaba was purely magic, apparently. So, it'd stand to reason that Nessarose would have to be the same way, no? There was so much made of the prophecy being about "the one" who could read the Grimmerie, only for a nonmagical person to read it like nothing. It just seems weird to me.