There are several languages that aren't part of ISO 639-2 but are part
of the 639-3 or 639-5. For those languages the legacy Matroska language
elements cannot be set to the ISO 639 alpha 3 code of the BCP 47
language tag.
However, there are a lot of such languages whose ISO 639 alpha 3 code
is a valid extlang subtag of a BCP 47 tag. For example: the language
"Yue Chinese" has an ISO 639 alpha 3 code of `yue` but isn't part of
ISO 639-2. However, `yue` is also a valid extlang.
As each extlang must have a prefix for which it is valid (in the case
of `yue` it's `zh`) and as that prefix must in turn be an ISO 639 code
itself, that prefix language's ISO 639-2 code is the closest
representation.
Part of the implementation of #3307.
