Raised when the language tag is invalid.
The language must be a valid IETF language tag (BCP 47); see RFC 5646
for syntax and examples. Tags are checked against the IANA language subtag
registry, but advisory findings — a deprecated tag or a redundant default
script (e.g. "art-lojban" or "gsw-Latn") — are accepted, since such tags
are still well-formed and reading systems handle them.