I have run into email templates not being available (the add user extension) and if there are no email templates available for the users language then an error presents itself. If using extensions and that ext requires a lang file the core of phpBB should always revert to at least the en directory of the extension as that lang is always required for a validated extension.

RFC topic http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=46771