This patch changed two things. The first is that for all IETF chapter
language elements that don't contain a valid ISO 639-2
code (e.g. because they contain a 639-5 code instead) a legacy chapter
language element set to `und` ("undetermined") is written. Not writing
a legacy element would mean that legacy parsers that don't support
IETF language elements have to use the legacy element's default value
`eng` ("English") in its absence, which is clearly not the language
the author intended.
The second change simply ensures that when multiple legacy or IETF
languages are present, each value will only be written once. This also
effects the situation above: when e.g. two IETF language elements
without 639-2 codes exist in the same chapter display element, only
one legacy element will be written, set to `und`.
Fixes#3174.
