We originally included it in T366181 but that didn't make sense because language codes in the request body aren't values, they're the keys in labels/descriptions/aliases. This applies to both language_code and site_id keys.
response code: 400
error code: invalid-key
message: "Invalid key '{key}' in '{json_pointer_to_parent}'"
context: { "path": "{json_pointer_to_parent}", "key": "{key}" }