I have subtitles turned off because I find them annoying. When playing the game I noticed two characters speaking in Japanese for quite a while and it seemed like I should understand what they were talking about.
Thus, I went ahead and enabled subtitles and lo and behold, Japanese was translated to English. Hence, I propose that even if subtitles are turned off, they should be enabled for any language but the chosen one. (Also, I think this is a bug.)

If you experience the same bug, please...

Subtitle OFF means no translation for any language