Zend_Translate_Adapter allows you to log untranslated messages if they are called, which is incredibly helpful. The trouble is that the priority level of these messages are hardcoded to the notice level. See lines 413, 426 and 799 (on 1.10.6). I would propose a logPriority option that would contain the log priority level in INT form. The lines would then be altered from

$this->_options['log']->notice("The language '{$locale}' has to be added before it can be used.");

into something like this

$this->_options['log']->log("The language '{$locale}' has to be added before it can be used.", $this->_options['logPriority ']);