This Windows app will enable you to download all subtitles for your library with few clicks.
It is using the www.opensubtitles.org library of subtitles for source.
Download and install the app and follow the provided step by step instructions.
I don't have an issuer certificate so Windows will warn you about the installation but click "More info" and "Run anyway". If you want, of course.. :)
If for some reason you want the source it's here.
Select your movie library folder, language you need, and let the app do the rest
You can continue unfinished jobs later.