Putting everything in their own folders isn't practical with as many movies as I have. The problem exists for TV series as well, where the total number of episodes in all of my shows is just too much.
The MC Rename, Move, & Copy Files (RM&CF) function is very easy to use, even though it has a bit of a learning curve. It can quickly move lots of movies and TV Shows to their own sub-directories. Sure, the actual move will take a while, but if it is just a move within the one disk drive, it will be fast. Having a very large number of files in one sub-directory on Windows is not a good thing. There is actually a limit to the number of files, though it is high for later versions of Windows, NTFS etc. Still, finding files in a sub-directory with lots of files is slower. It just isn't best practice to put all movie or TV Show files in one sub-directory.
Also, I believe this is some kind of root cause of another problem I've had, where language identifiers aren't recognized. So it isn't really helped by separating the folders, because MC seems to completely ignore the filename of the subtitles and thus doesn't care about the language identifier.
I haven't tested MC's language identification from the file name for some time, so any Subtitle files I collect are assumed to be English, and are offered for selection. But it used to work, so I would assume that it still does. Language identification on srt files isn't as good as other formats, because it does rely on the file name rather than have the language information embedded in the file. But it works. I'm pretty sure MC only uses the language identifier for Force Subtitles anyway, so that the correct language Forced Subtitles are displayed automatically.
May I ask what the _OS_ in your subtitle filename is? I thought the "identifier" was for languages only. Also, your "English" is different from the "en" I've been told to use. Is there some hidden Wiki page that explains this?
I believe that the "OS" indicates that MC got the Subtitles from OpenSubtitles.org. The suffix can have more than just the language identifier in it. "English", "en" and I think "eng" are acceptable suffixes to identify the English language for the Subtitles. There isn't any specific JRiver Wiki describing the file naming layout for suffixes. I learned that by Googling and reading many years ago. Unfortunately, I have never found one reference site that provides all information about SRT naming and formatting.
I did just download French and Italian Subtitles for "The Matrix" using MC, and the files were named "The.Matrix.1998.Bluray.1080p.BluRay.x264_OS_French.srt" and "The.Matrix.1998.Bluray.1080p.BluRay.x264_OS_Italian.srt" respectively. MC then offered the English, French, and Italian Subtitles for selection.
If MC doesn't appear to be recognising the language in your Subtitle filenames, try using "English" instead of "en" and see if that works. But as above, it is only used for Forced Subtitles, allowing you to select other languages when you want to, even if your default Audio and Subtitle language is English. SRT files do not support Forced Subtitles.