See https://forums.opera.com/topic/28138/a-problem-with-downloading-and-activating-another-spelling-checker-in-opera-mail.

That thread points to https://extensions.openoffice.org/en/project/czech-dictionary-pack-ceske-slovniky-cs-cz, but you can search the site for a Norwegian one to see if it works. https://extensions.openoffice.org/en/project/norwegian-dictionaries-spell-checker-thesaurus-and-hyphenation might be it, but didn't check.