You can set your preference for language(s) of subtitles under
(rightclick >) Options > Preferences > Subtitles [Subtitle properties]
Last option in BS.Player's window (middle 'column') "Default subtitle language".
Changing this setting may be buggy in your version/build, but that problem as well as the problem of the pre-checked boxes not being only in your preferred language(s) is repaired in latest
beta (version/build 2.57.1045).
(Sofar it wasn't repaired in any released regular build yet.)
So please download and try most recent beta version.
(If you're a Pro-user you can download from your
user-area and if you're a Free user you can download from BS.Player's
website.)
P.S.: Please when posting allways mention BS.Player version/build number!!