Within the show's mythology all the names supposedly derive from a time at least 500 years ago, To follow your example, (just in English) cat, catt, catte, Kat, Katt, and Katte all refet to the purring, string player and can be found in written material from the late middle ages and the early modern period .It is only within the last 150 - 200 years that spelling has become standardised in western alphabet writing. The spelling before standardization was entirely based on the sound of the word and so varied from region to region based on the local dialect, so an English speaker in Germany would spell a term entirely according to his/her own phonetic rules. The Griim Diaries and Rosalee's books are secret writings passed on through family and Master/Apprentice lines, not for general publication. It is entirely possible that the spellings found in any material owned by Maya and inherited from her ancestors would differ from that in Nick's library. Furthermore since the world of a Grimm is by necessity hidden, the names being almost correct, but in fact nonsensical means that they are likely to be discounted if discovered by the uninitiated, further protecting the secret.