I'm organizing my songs by language and this happens from time to time. I'm thinking of using the deprecated "JI"[0] but would love to follow a standard if there is one.<p>[0]https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes#JI
Consider using the ISO 639-2 three-letter code or its extension ISO 639-3. They have codes for “miscellaneous”, “undetermined” and “no linguistic content” (<a href="https://en.wikipedia.org/wiki/ISO_639-2#Special_situations" rel="nofollow">https://en.wikipedia.org/wiki/ISO_639-2#Special_situations</a>) and a range “reserved for local use” (<a href="https://en.wikipedia.org/wiki/ISO_639-2#Reserved_for_local_use" rel="nofollow">https://en.wikipedia.org/wiki/ISO_639-2#Reserved_for_local_u...</a>)
In one of my past employers, we used “ZZ” for the “gibberish” environment, which was auto generated from English EN to include all sorts of accents and symbols, as well as padding to the max width specified on every label and field.