In response to "Translation support for plugins" :
In the core there could be some variable of the locale chosen language (en_US, pl_PL, etc.) - it will be good for plugins that uses the language as a parameter (e.g. facebook login, like button).
My sample code :
<?php
function GetLocale() {
$locale = str_replace('-', '_', substr($_SERVER['HTTP_ACCEPT_LANGUAGE'], 0, 5));
if (strlen($locale) < 5) return 'en_US';
if (strrpos($locale, ','))
$locale = substr($locale, 0, 2) . '_' . strtoupper(substr($locale, 0, 2));
return strtolower(substr($locale, 0, 2)) . strtoupper(substr($locale, 2, 3));
}
?>