Per provider language
Some providers like opensubtitles adds machine translations very early, and this leads to bazar download bad subtitle. It's known that some providers provide better subtitle for specific languages and bad for others.
opensubtitles provide better subtitles for english, but very bad for Bulgarian
subsunacs provides better subtitles for Bulgarian, but english could missing in most cases
Bazarr's providers declare which languages they are supporting, so the easiest way to got this is to override the provider's language list before doing to download. I know it would require manual touch on every provider (usually they do that in provider's constructors), but if there was an API for that I could help.