Changing language within a theme?

I have installed WordPress in Norwegian. However, the theme I’m using (A Beautiful Theme from Studiopress) is in English.

As far as I can see, all I need to change is in the blog entry title where it says “by (author)”, “Comments” and the search widget where it says “Search this website”.

How do I do this?

Website: https://www.camillaandersen.no