It depends on how you analyse the word "kristna". In the first sentence it's an adjective, which means that the word describing it has to be an adverb. In the second one, it's a noun, and hence the word describing it is an adjective.
I'd say that the former is slightly more on the prescriptivist side and the latter a tiny bit more relaxed, but unless you're using a really formal register, they're interchangeable.
read fluently, understand well, speak badly;
read fluently, understand badly, can't speak;
read some, understand a bit, speak a few sentences;
heritage language, want to understand and speak but can't.