Rectification: you don't need to convert them, if you have subtitles with the same name as your movie name, they are correctly loaded (by VobSub, not yet tried with BSplayer internal VobSub feature)... But they still are in Japanese
(Note, rename the both of .idx and .sub files, finally you should have these three files in the same directory:
bien-xvid-ff7.avi
bien-xvid-ff7.idx
bien-xvid-ff7.sub)