"You are good at Norwegian."
Translation:Du er flink i norsk!
In Norwegian one is "flink i" specific languages and school subjects, and "flink til (+ infinitive)" skills like reading, writing, and cooking.
"flink på" is only used in a select few cases, like "flink på skolen" for "good at school".
Prepositions can be pretty tricky when you're starting out learning a new language, but you'll get the hang of them eventually! :)