Try
Bazarr. Should work as stand-alone, but not sure on that.
It works best with Radarr and/or Sonarr (These can be used without downloading off Torrent sites!), as it synchronizes with them and download missing subs according to your preferences.
It can use multiple download sources, and it can also fix/modify/sync with existing media. Very potent little program this one.
Make sure to register on the subtitle sites for more or full access.
Edit 2023-03-04:
There's also a version of both Radarr & Sonarr that can use AirDC++ as download client, although they can't be updated.
Most likely updating has seized due to lack of interests.
It does still work though.
Check it out
RadarrAirDCPP or
SonarrAirDCPP 