Can you give an example of how you have your files structured and named?
As long as the subtitle and video file are named exactly the same (excluding the extension obviously, and the language code as @tom80H mentioned), they should show up in Plex.
For instance on my test server where I have WandaVision, I have files named like such:
Note I had to change from using SDH to CC because SDH was being recognized as a language code.
As you can see, when I have them named without the language code (as I never had the year in the filename to begin with) they still show up but Plex just does not know which language they are.
Regarding renaming, there are tools out there that allow for mass renaming, such a FileBot (which is cross platform) or TinyMediaManager (also cross platform). Either of those should be able to assist you in renaming the subtitles and possibly your video files so that they match.
The implementation Plex has is fairly standard as even VLC uses that to auto pick up external subtitles and know what language they are.
adding year to movies/series is to help match the movies/series. not the subtitles.
the subtitles should be named the same as the movie/episode files, and as indicated above should show up as UNKNOWN.
if you want them to show as the actual language of the subtitle, then you need to add the applicable language code in between the filename and extension.
long story short, your subtitles should already be detected as UNKNOWN, and if they are not, then you possibly have some permissions issues (plex can’t read them), or some other issue.