Make Custom GPT response in a specific language and response with custom database


I have created a custom GPT by instructing it to response with Classical Chinese no matter what language is being asked and only answer base on Classical Chinese Classics. But after a few prompts or quite randomly, it starts to respond with modern Chinese or, very rarely, English (my prompt are mostly in modern literary Chinese).

Every time, when it quote or cite something, it was almost always wrong (either no such thing, or correct phrase paired with wrong author/book). I’m interested in update it with some Classical Chinese classics (Si Shu Wu Jing Zuo Zhuan the 24 histories etc.). My question is what would be a better way to import these knowledges. It going to be GBs in pdfs, or hundreds MBs in txts. There is also public domain resources on ZH Wikisource. One problem is how to let GPT know the word in a txt/pdf written by a particular people/its book title/how it referred in other Classics. And how can I instruct GPT to answer only with the language in those Classics.

Thank you very much.