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” (https://en.wikipedia.org/wiki/ISO_639-2#Special_situations) and a range “reserved for local use” (https://en.wikipedia.org/wiki/ISO_639-2#Reserved_for_local_u...)
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.
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” (https://en.wikipedia.org/wiki/ISO_639-2#Special_situations) and a range “reserved for local use” (https://en.wikipedia.org/wiki/ISO_639-2#Reserved_for_local_u...)