There definitely is a tagging convention; ffmpeg / ffprobe treat language as a tag that applies to every media stream. If you're creating or editing a file, you can set that tag to whatever arbitrary value you want.
I believe there's supposed to be a convention of using shortcut codes for languages, but when I create files I intentionally violate that. I use mpv to play videos and when you select an audio or subtitle stream, it will display the language tagged for that stream; I prefer to display "English" than "eng" or "en".