The letters with unknown associations, in alphabetical order, are: B, G, J, O, Q, V, X, and Z. This is because these letters do not appear in the three sentences Paprika has in story mode chapter 1. If Paprika were to use more Papdings in the future, these symbols might have become known. But since production of the game has halted and the publisher fired the developers, we may never know.
You may or may not know this, but Paprika’s lines in chapter 1 have already been translated. They are, in order: