For translation we need either:
-The Japanese subtitles (very hard to find).
-The script, manga or novel that the movie was adapted from.
-Other language subs, most of the time Chinese (common).
How do you find Chinese subs?
-Get the Japanese title of the movie from Asianwiki.com or a Google search, a Wikipedia search.
-Here too you can type the English title or the name of an actor to search the title in Japanese, or better, in Chinese: MTime.com