FWIW, this is implemented in VLC (and likely many other media players) simply by reading the title metadata field from each track, both audio and subtitle. When backing up a dvd using dvdbackup on Linux, commentary tracks are often pre-populated with title=Director's Commentary in the metadata for the appropriate tracks in the VTS_0X_0.IFO file.
My backup procedure preserves this information throughout the encoding and muxing process, it’s available for use to Plex, but Plex does not use it )=
Here’s what mediainfo says about one such track from Fight Club:
Audio #3 ID : 4 Format : AAC Format/Info : Advanced Audio Codec Format profile : LC Codec ID : 40 Duration : 2h 19mn Bit rate mode : Constant Bit rate : 161 Kbps Channel(s) : 2 channels Channel positions : Front: L R Sampling rate : 48.0 KHz Frame rate : 46.875 fps (1024 spf) Compression mode : Lossy Stream size : 161 MiB (9%) Title : Director's Commentary Language : English Default : No Alternate group : 1 Encoded date : UTC 1904-01-01 00:00:00 Tagged date : UTC 1904-01-01 00:00:00 Menus : 11
And here is how VLC presents all of the audio tracks in Fight Club:
