@apistoletov said in Is there a more-or-less standard convention for storing Language info in the (FLAC) tags?:

On the other hand, it's somewhat common to have songs in made-up languages which wouldn't have the language code, but is there any interest in supporting them?

You could use "mis", "und" or "zxx", depending on the case
https://en.wikipedia.org/wiki/ISO_639-3#Generic_codes