ConsoleLanguageForTag picks, among the rows of the tag's language whose script
and region are empty or equal to the tag's, the one that names the most
(script outranks region), so a regional row wins over the language's main row
and a language with no row for the tag's region falls back to its main row.
The first group of rows covers the Latin-script European languages that have
no regional split but a few: English (United Kingdom for GB, IE, AU and NZ,
English (United States) otherwise), French and French (Canada), German,
Italian, Dutch and Romanian.
ParseLocale normalises the text as a POSIX name first, which leaves a BCP-47
tag unchanged except for underscores, and then parses the result, so the
caller handles en-US and en_US.UTF-8 alike. C, POSIX and an empty text report
no locale and leave the output tag as it was.
POSIX hosts name their locale language[_territory][.codeset][@modifier], such
as en_GB.UTF-8@euro, and use C and POSIX for no locale at all. NormalizePosixLocale
drops the codeset and the modifier, turns underscores into hyphens, maps the
@latin and @cyrillic modifiers to the Latn and Cyrl scripts where the name has
no script, and returns an empty string for C, POSIX and an empty name so the
caller sees "no locale" instead of a language called C.
ANYPS5_LANGUAGE can only name a console language number. Taking the language
from the host needs the host's locale, which arrives as a BCP-47 tag such as
en-US, pt-BR or zh-Hant-TW. ParseTag reads the language, script and region
subtags, normalises their case, ignores variants and extensions, and rejects
empty subtags, subtags over 8 characters and languages that are not 2 or 3
letters.
MarcZX10