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...

ok, got it, found out that the first string has to be the name of the lang file... my fault...