If you use this method then I have the perfect solution. the missing piece I was looking for so long!
In fact you have to find a way on how to trigger a) language related messages from add-ons, and b) language related messages AND emails from expressionengine itself.
you can do that by adding two variables within the index.php file, just before "END OF USER CONFIGURABLE SETTINGS":
// $assign_to_config['language'] = 'deutsch';
(attention, when activating this, you have to translate all of your add-ons, or at least, duplicate the language folders. )
// overrides the default language for expressionengine (use the name of the language folder)
$assign_to_config['deft_lang'] = 'deutsch';
this is much more important and will set the language of system messages and emails!
(for multilanguage emails you have to go to: "design -> message pages -> email notifications" and de-check the checkbox for each template: "Enable This Template? If disabled, this template will not be used. Instead, the message will be generated dynamically in each user's chosen language"