Never mind. Finally found exactly what I was looking for. If you set the languages, check ‘Automatically select audio and subtitle tracks’ and THEN set Prefer Audio Tracks to NONE, it will not change the audio defaults. Could this be moved to the top of the dropdown list, for much better discoverability? Thankyou Plex for surprising me 
Right now, there is a major flaw with the handling of default subs from existing files. If I look at my movie collection, I have a movie with the following tracks and following default settings for those tracks (set within the MKV file itself)
File Format: MKV
Video Track: English Default
Audio Track: English Default
Subtitle Track 1: English Default & Forced (for foreign subtitle parts)
Subtitle Track 2: English (no default/forced)
Plex does the right thing here without the ‘Automatically select audio and subtitle tracks’ option ticked, and uses Subtitle Track 1 by default (respecting the ‘Forced’ flag), without the user having to manually select it.
On the other hand, I have a file with the following tracks as a TV Episode (using HAMA and Absolute Series Scanner, this should not affect the language settings at all)
File Format: MKV
Video Track: Japanese Default
Audio Track 1: English Default
Audio Track 2: Japanese (no default/forced)
Subtitle Track 1: English Default & Forced (for foreign parts within the video itself)
Subtitle Track 2: English (no default/forced)
Plex does NOT select ANY subtitle Tracks. It should be selecting Subtitle Track 1 by default, respecting the settings within the MKV file by default, but it instead ignores the default/forced flags and selects nothing.
If I enable ‘Automatically select audio and subtitle tracks’, then it selects Subtitle Track 1 (respecting the ‘Forced’ flag) for this file. Which then brings up a new major issue. Plex doesn’t have an option to only select Subtitles for you, it will also always change the default audio language..
Because of this, if you have a file with the reverse default settings, it will ignore that your audio language preferences and select the English language anyway.
This is a major issue when it comes to animated content from foreign countries (Anime for example). Manually selecting the subtitle track for up to 24 files (and in some cases, hundreds of files) PER SHOW goes against the spirit of Plex. Without options, it becomes a struggle to selective curate the default language of different shows (some have good English dubs, some have horrible ones).
Can we please have an option to always respect the subtitle language defaults in the files themselves, or otherwise an option for Plex to only select subtitles, and not change any language defaults for Audio tracks?
