Seems no matter what I try there is no way to only ever automatically activate forced subtitles.
Due to many video files having “unknown” audio and that plex seems to treat that as foreign audio many videos are getting subs enabled needlessly, and thus forcing transcoding on many clients.
There need to be an option to ignore unknown/undefined audio or assume is the selected language. Or an option to only auto enable subs for forced subs.
If you are downloading all of your content, you might want to analyze it yourself and fix it before adding it to your library. I use SubZero and it only grabs forced subtitles and they are enabled automatically - but I rip all of my own content, so I know exactly what the primary audio track is and Plex does too.
This is still a thing… There still needs to be an option for “Assume Unknown Audio is ___________” or just an option “Do not consider Unknown Audio as Foreign Audio”, this is stupid that I should have to remux/edit a vast majority of files just to prevent them from displaying subtitles automatically, while still having forced subtitles show correctly.
Tweeted at them, got the “Working as intended” reply. 
This is a rediculus feature.
“Sure we auto enable forced subtiles, but only if you like subtitles on the other 99% of stuff that dosnt need them”
I do see your issue and it does suck that Plex classifies undetermined as unknown.
It be nice if Plex just use your local language for undetermined.
Something like… Undetermined(Using Preferred Audio Settings).
Anyway, the only option you really have is to set the tracks yourself. I’m not sure how you go about remuxing files but in any case, here are 2 small batch snippet to set MP4/MKV audio tracks to English.
I use them quite often. Maybe you or someone else will find them useful.
_MP4_EditAudioLanguage.cmd requires MP4Box. You can download the installation program here
The only component you need is MP4Box. You can un-select all the rest.
_MKV_EditAudioLanguage.cmd requires MKVToolNix. You can download the installation program here
Simply run the command file(.CMD) in the same directory your MKV/MP4 files are located at.
_MP4_EditAudioLanguage.cmd will set track 2(this is assumed as the audio track).
This will write a temp file then replace the old with the newly created one.
_MKV_EditAudioLanguage.cmd will set the first audio track.
This will modified the MKV file header. The process is almost instant.
Thanks Ill take a look at that, I already use MKVToolNixGUI to do it currently, even converting mp4 to mkv in the process (without transcoding), only thing is, I have 13k/14k files, and a vast vast majority have to be “touched”, to overcome this rather stupid limitation that is “working as designed”.
Sadly I have come to realize plex does what plex wants to do and damn anything else.
AGREED!!! I have nearly 23,000 (twenty-three thousand) TV episodes which include Metadata, artwork, etc – the only thing they do not have added… is the audio language set!! YIKES!!
Manually turning off the subtitles every night, several times a night when I watch “TV” is ludicrous.
If a language is set to UNKNOWN, then (I believe) Plex should do NOTHING and assume it is in the Players Native language.
The worst part is when Plex retrieves bogus subtitles from the internet which are jarbled and only include ads or mumbo jumbo instead of actual subtitles/captions of the content being viewed.
This really should be changed by Plex Inc.
At the very least, Plex should modify the Subtitle Setting in the Plex Player app(s) - I use Android, to include Manual.
