I read the canon for my Yuletide story last year in the original language (Japanese) and it was interesting and frustrating trying to capture the characters' speech styles in English. A few times I came up with dialogue in Japanese and then translated it directly, other times I would write the English first and then translate it in my head to see if it 'sounded' the same.

I didn't worry so much about the narration, because manga is visual anyway, and text isn't. But one thing I was concerned about was not throwing in too many untranslated Japanese words, because I didn't want exoticism, or to contribute to pernicious language myths.