Porting to 2.5.x loading en-gb language first

This port the same PR that was merged into 3.2.x
It lets load en-GB language files before other language packs to cope for missing strings.
To test, delete some strings from any non en-GB language file and display the results, in admin or site.
Instead of the constant, one will get the English (en-GB) value when debug is off.

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33354&start=0