[feature/twig] Remove get_lang function (it's not used anywhere)

PHPBB3-11598
[feature/twig] Don't forget to set the context when rendering!

PHPBB3-11598
[feature/twig] Clean up the messenger a little bit

This should fix at least one bug, noticed we were using:
$this->vars = &$this->tpl_obj->_rootref;

Which hasn't been valid for a long+ time

PHPBB3-11598