But you have to have more data than used for pretraining for the added knowledge to take precedent over pretraining, no? If that would be the case, you practically contradict the knowledge in the base model.

I mean ... LLMs are sort of an extreme and living proof of linguistic determinism. Their behaviors are dictated almost entirely by disorganized language data, primarily English and Chinese. So you can't just add a language as native primary language in a quick post training, I think. There's no way that it would work.