Are you happy to read in English? I enjoyed this Fijian mystery series as you get a real feel for what it's like to live there (I have visited, and perhaps you did when you went to Samoa). There are five books in the series, plus a prequel story about how the main character, Joe Horseman, became interested in detective work. I really enjoyed them because they tell you a lot about the country and culture. The author is Australian, but lived there a long time. Of course they are all murders which I find sad, but I like the solving of the mysteries and the island setting.

I haven't read any of those northern ones, but I've heard they're depressing, like the "nordic noir" tv series set there.

[edit] I see you want Japanese translations, and I don't think these would be popular enough to have any. What a pity.