Thanks @winterborn for doing most of the work here.
Downloading/updating filters
To download filters for a specific language use --update --lang=de, in this case de being the 2 character ISO 639-1 language code.
Once a filter has been downloaded it will automatically be updated when using --update. If a filter fails to update because of a 404 error the file will be deleted.
Cleaning subtitles
When cleaning the language code will be extracted from the file name. If your file is subtitle.de.srt then de-main will be loaded along with main. If you would like to force a language file to be used, you can specify using the --lang parameter.
For example: subclean subtitle.srt --lang=de. Once more filters have been added you will be able to pass multiple language codes at once, for example: --lang=en,de,ja.
If a language code is invalid it will be ignored.
Contributing
The filters rely entirely on user contributions, if you find ads in your subtitle files please report them on GitHub so other users can benefit from the new filters. Right now I'm adding new filters as I find them so I would appreciate all the help with other languages.