Pass the language code to the MessagesPreLoad hook

Pass the language code to the MessagesPreLoad hook

So that extensions like MessageCommons can try to use this (and
$wgLanguageCode) instead of $wgLang->getCode()/$wgContLang->getCode(), as
the latter ones cause fatals and recursion, at least with the Gadgets
extension also enabled at the same time.

Change-Id: If71fe1ded26c7a1c771128397783783ad5715b00