libxml2 has a problem when a document encoding is specified, even though
documentation says that all internal representation would be in UTF-8 and
language-subtag-registry data already is passed in UTF-8, when specifying the
document encoding to be UTF-8 the output to language-subtag-registry.xml is
conversion garbage, '?' on Linux UTF-8 locale, ISO-8859-1 on Windows. If no
document encoding is specified at all no conversion happens and non-ASCII
values are correctly written as entities.