Language file is not loaded

Hey folks,
maybe I missed something, but my language file called App.php (in app/Language/de/App.php) is not loading. When I reach the login page with myth auth... language file has been loaded (APPPATH/ThirdParty/myth-auth/src/Language/de/Auth.php).
My understanding was, that CODEIGNITER will load the language file automatically... DID I MISS SOMETHING? If i check the language with ($this->request->getLocale()he result is "de", so language setting is correct.
Don't laugh, I'm searching now for hours...