Weird audio codec display

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If you can figure out where this and the sub info is being scraped from, I'll pay for your Jellyfin subscription. Been driving me a little mad as well. I screwed up a couple batch encodes and despite the actual info not existing in the file via ffprobe or mediainfo as far as I can tell, Jellyfin has the correct info for the audio track, just with my wrong metadata before it:

Opus 5.1 Surround - English - German

I copied a template I've been using for stripping non-English streams and used it for a German-only show. Corrected everything except the metadata info. Somehow Jellyfin knows it's German despite me removing that information.

I'm more familiar with ffmpeg than Mkvtoolnix, but there's some crossover. The ffmpeg "tag" flag forces a video or audio stream tag, what ffmpeg calls a "fourcc" in their documentation which isn't really explained anywhere that I could see. That's rampant in the ffmpeg docs. From context, I'm guessing fourcc is Four Character Codec designation: opus, hevc, etc... Finding the list should be as easy as ffmpeg -codecs. I'm guessing putting the correct fourcc in for each track would get you closer to what you want. Might be the same with Mkvtoolnix -- figuring out exactly what field needs to contain the info and how to format it.