HaikuDepot : Improve Language Code Handling

HDS is soon going to be enhanced to deal with more than
just the two-character ISO language codes that it
currently supports. The logic in HD is currently expecting
that HDS will only supply these two character codes. This
change is about making adjustments to be able to cater to
any ICU language IDs from HDS.

The naming in HDS is still going to continue to use the
terminology that it currently does; it will call the
identifier for a language a "natural language code". The
HD source will instead reflect the nomenclature of ICU
and call the same thing a language ID.

Change-Id: Ib138c2e3b7b667edf15cd497c2f710f80b3c29f8
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7424
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Niels Sascha Reedijk <niels.reedijk@gmail.com>