Subtitles

I don't know about Heavenly Creatures. It's been too long since I seen the Peter Jackson psychological thriller.

But it's not unheard of that a director doesn't have a translation of everything to help the audience know everything that's going on in a given scene or movie. Not translating some foreign language lines could indicate that a key character in a given scene also doesn't speak that nonEnglish language being spoken in that scene. Not knowing more then that character helps you empathize with he or she a bit more.

Also, of course, if the dialogue in the foreign language really has no bearing on the plot but is inserted only for authenticity, there's no need for a translation.

Good point. Many times, dialogue and lyrics from either background televisions or music from radios don't get English/translation subtitles because they're not the focus of a given scene or story.