I think it would be helpful if we can actually ‘tag’ the audio language for an entire tv show and add it as metadata (the same way we can upload a poster or change the summary of a show/movie), which won’t actually change the file. And without having to use a third-party editor to change the audio language.
A lot of videos have ‘unknown’ as the audio language which could cause some issues.
Most TV Shows have the same audio languages for all episodes anyway.
This already exists, you can already tag the language for the audio tracks in an mp4 container., and Plex will pick it up from there. You can batch remux/tag other stuff (avi, mkv) into an mp4 container. That way the files also work correctly outside of Plex.
I’d like to have the option inside of Plex.
I have an issue with +300 files from a certain TV Show (and probably more that I haven’t even noticed yet), tagging each file separately with an app like MKVToolNix is enormously time consuming.
If Plex would make it possible to change the language tag in the metadata of an entire TV Show that would save me hours of work and would limit the amount of human errors.
From experience I’d guess it’ll take something between 30-45 minutes max, manually fixing those (raises the question why you didn’t fix it right away when you digitalized your video library).
That being said (and probably deemed “too much”)… there’s apps that’ll do that for you.
You probably can spend the same amount of time to research it 
(sorry… couldn’t resist)
I googled options, but I’m not a developer and the options I found are not really user friendly since most of them require the use of command lines (when I tried to do this this, I messed up the audio of -thankfully- copies of my video files).
I am changing the tags manually for each file with the MKVToolNix app (with homebrew), but for the MP4 files, I have to convert them first to MKV with Handbrake (which is time-consuming and a bit harder on my device).
So this option would be helpful in the future and also for people that don’t have the technical knowledge to change the tags with third-party applications and command lines.
You can just drag your mp4 file in MKVToolNix and remux it to an MKV container. No need to transcode it using Handbrake.
Alternatively there’s similar apps for mp4 – the link above mentions a few requiring a command line; there’s others working with a graphical user interface (I’m using Subler on macOS; you can e.g. use videohelp.com to search for apps to mux/edit mp4 or other container formats on your OS).
Oh, that will probably save me a few hours 
when I imported it into the header editor I just got a warning they could only process mkv-files.
I’ll also look into Subler.
Altho I’m a bit cautious to change metadata in my original video files.
The header editor will probably have that restriction.
Use the Multi-Plexer tab, drop your file and click “Start Multiplexing”. You can directly check the language tag for the copied streams in the multiplexer (Tracks, chapters and tags box).
I’m quite at home in the terminal, and I use this approach often for minor fixes to archival video in my collection (digitised VHS transfers). However, the reason I use Plex is to be able to make adjustments to my metadata without the risk of a dodgy regex destroying my files. If a feature as eminently useless as editing the writers on a movie is possible, the ability to add language codes to audio streams (including tagging commentary tracks) isn’t unreasonable.
