In case you don't know, Norwegian and Swedish are kinda similar languages, so knowing one language can help you understand the other, to some limit. Also, Finland is bilingual and it's mandatory to learn Swedish in school. Katariina's job also requires her to know some Swedish.

(Jousia knows quite a few languages because he finds it interesting and useful. Nilsa lives next to the Swedish border and pretty close to Norway as well, so he's adequate with the languages.)

That's a small language info bit for you! Have a nice weekend!