The supplementary audio descriptor is defined in ETSI EN 300 468 and
provides more details regarding accessibility audio tracks, especially
the normative annex J contains a detailed description of its use.

Its language code (if present) overrides the language code of an also
present ISO 639 language descriptor.

Note that this also changes the priority of multiple descriptors with
language from "the last descriptor with language within the ES loop" to
"specific descriptor over general ISO 639 descriptor".
Signed-off-by: Aman Gupta <aman@tmm1.net>