the Joomla! language override system is not meant to be used to translate to another language. You can use it, but it will be very painful. The usual way is to look/ask for an appropriate language package.
I assume, You were looking for language strings from the emails (confirmation, some special texts of the newsletter). I also assume, You were looking for these language strings in the front end language files. But these strings reside at the backend language files, because the mails are composed ans sent from the backend.