The 'watch folders' are not really intended for what you're trying to do. It will probably work but perhaps not as you expect.
For example, if you search for multiple languages and it finds a video that already has a single subtitle, it simply skips it. No matter if there are languages missing.
Also, it doesn't use fsevents so it truly scans your folder tree every time. This can become quite slow.
The watch folders are intended to be used to 'watch' one or more download folders. So if it sees something new coming in, it keeps an eye on it. And if it thinks it's done downloading, it searches for subtitles and then doesn't look at it anymore. Assuming you will move the video to it's final destination at some point.
I've got a few more requests for what you're trying to do. It's on the list but it's quite a challenge to do it right so I need to think of a way to do this properly.