This makes certain (possibly wrong) assumptions about your system, but if you have CCCP installed:
Start Menu > Haali Media Center > Haali Media Center Setting
In the "Audio and Subtitle Preferences" field, enter* jpn,jpn;jpn,eng
Set "Match subtitle track name to audio" to "Yes"
This should work for
most dual-audio releases out there, and it will work for Arigatou's** Blood+, as it only has one subtitle stream.
*Some groups tag the full subtitles as "Japanese" and the Songs/Signs-only subs as "English" -- instructing Haali to look for jpn subtitles first will cover those releases as well as ones that tag the subs as English.
Note that if you have a release where both subtitle tracks are tagged English, and the first one is the Songs/Signs-only track, you
will need to manually change that, with
perhaps one exception.**Though I must ask, if you want to watch Blood+ in Japanese, why are you watching the Arigatou version? Those particular R1 DVDs were dubtitled, meaning the subs are straight transcripts of the English dub. If you want accurate translations, I suggest getting the Shinsen TV-rips, or the
ecchilicious DVD version that uses Shinsen's subs.