Most classes are taught in Norwegian. However, many schools offer Norwegian courses, and nearly all staff members and students speak English, which can be helpful in the beginning.
After a year at a folk high school, you can expect to understand and speak Norwegian reasonably well. Your level of proficiency will of course depend on your own efforts, but living in a Norwegian-speaking environment gives you great advantage when learning the language.
At some schools, you may select Norwegian language and culture as a major subject or as an elective.
If you choose Norwegian language as an elective, you are free to choose between the other major subjects offered.