Forced Subs are a function specifically for “foreign speaking parts only” and are a function on discs so that they don’t have to hardcode those subtitles and can use vector graphic based text so it’ll look good on any size screen. It’s not a separate subtitle track on the disc though; what happens is there is a flag on the regular subtitles to say “force this text to show even if subtitles are turned off”. There is only one subtitle track per language but the disc\player can read the code in the file to pull from the right subtitle track for the right language selections and provide full or “foreign parts only” from the same subtitle track.
When ripping discs and creating a forced subs track only those flagged parts are pulled from the full subtitle file as it’s own dedicated subtitle track. Then a separate subtitle track can be included with full subs. You have to pick one or the other; often the only subtitle track on a rip, depending on the group (I’m assuming you aren’t ripping your own), is the forced subs.
It’s a function that’s built into the disc playback that doesn’t transfer to digital files when ripping discs so you end up with multiple sub tracks and having to manage these things a bit more to make up for it.
Plus different ways of ripping and setting up subtitle files creates different results - if you don’t flag the tracks correctly the player has no way to know language of the tracks or forced\default behavior.
So your experience and options will depend a lot on the rips themselves and the subtitles available and the player you’re using and so forth…
In Plex you can set some default language settings for the usual circumstances and that covers the majority of situations for most people as long as the tracks are labelled correctly. There’s no way to really account for multiple language preferences well so it’s a limitation of functionality so in some cases you have to pick and choose manually sometimes.
Recently Plex added options to set a different audio\subtitle selection than account defaults per TV show and have it stick for all episodes of the show without having to manually change each episode; this is useful for multi-language files where you might want something played back in something other than your preferred language - common with watching anime - or the language\subtitle tracks weren’t labelled right so can’t be picked up by the automatic function.
You can also set your account to always playback the full subs if available if that’s your preference … that’s in the language settings for auto-select mode.
If your rips don’t have the subtitles you want you can often find them at sites like subscene.com and use the article about manually setting them up in Plex so they’ll be available and labelled correctly. https://support.plex.tv/articles/200471133-adding-local-subtitles-to-your-media/
Plex\Jellyfin\Emby\Etc are just code running on machines (Roku\Chromecast\AmzFire\AppleTV\Etc) and are limited to some degree because of the nature of media files ripped form discs or streaming services being played back on those devices; companies involved in media distribution and those STB devices don’t really support that situation.
Streaming services also manage subs\audio based on your default account settings - and if they don’t offer specific language options for audio or subs you’d not have them there either (and they region lock some of that stuff). So if you want a solution that doesn’t feel “so stupid” and has the most compatible and supported functionality then you should stick with discs and disc players for the most complete and seamless solution for language support and language options.
