I finished this project. as it turns out, multi-page pop up menus
are the way to go when doing blu-rays.
To answer your question, yes you can enable the sub-titles while
the movie plays.
you select the sub-title button, then it’s properties, click specify link.
choose the sub-title button, then only switch the sub-title (lower right)
to the one you want.
It will then enable the sub titles while the movie plays and should
remember it from then on.
I had needed that as well for my project, but instead of sub-titles, it
was for switching to audio commentary.
blue streak is still in business and quickly answer emails.