3 letters language code issue when installing Joomla

[infograf768]

As you have noticed, our Joomla Installer chokes on 3 letters iso code
languages.
See :
http://groups.google.com/group/joomlabugsquad/browse_thread/thread/e7188716859b3b17?hl=en#
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=16112

I guess this may be out of the scope for 1.5, but I propose the
following solution for 1.6.

For this solution to work, we need to use one of the languages
accepted by browsers.
The list is here: http://www.brandonitconsulting.co.uk/mod_metamod/language-codes/
(Thanks Klas)

In the same way as we had a windows backward metadata param in the
language xml (that one is now useless for 1.6)
<backwardLang>english</backwardLang>
we could have a metadata stating a 2 letters iso code replacing the 3
letters one when installing.

In the case exposed in the above tracker, the language is sla-ME
we would have
<browserLang>sr</browserLang>
as Serbian is the nearest available browser language for sla-ME.
The metadata would only be present when using a 3 letters language.

The Translator would pick his preferred default browser language in
the list.

This is just needed when installing Joomla as 3 letters languages
packs are installable through Joomla extensions installer and create
no issue in all Joomla behaviour.