As always, it depends.
Plex can automatically activate “forced” subtitle tracks.
You set up your language preferences under https://app.plex.tv/desktop/#!/settings/account
like so:
This preference is separate for each plex.tv user account and also each “managed” user.
The video file must have the forced subtitles as a separate stream.
The stream must be marked properly as “forced” (only possible in MKV files). Merely naming the stream as forced is not sufficient. It must be the dedicated “forced” flag.
Additionally, both the audio and the subtitle streams must be marked with their respective language information.
The subtitle stream should be in SRT format, as this format is text-based and thus is more likely to be playable without transcoding. Introduction: convert image-based subtitles to SRT with Subtitle Edit
Subtitle format support is dependent on the type of player device.
If the video file has an audio stream which is not compatible with the playback device or is incompatible with the attached speaker configuration, Plex server may still transcode the full file to avoid synchronity issues between video and audio.
So in the ideal case you have at least two audio tracks in your file: the first in AAC stereo and the second one in the original surround sound format - whatever that is.
See this forum section for other related tips https://forums.plex.tv/c/general/tips-tricks-how-tos/15
