Hello,
well the only other option would be to redefine INVOICE language constant to "Rechung und Lieferschein".
The only way to change this is to work with the language translations for the component as you did: either change the existing or update the code and define a new one.
Just be careful with updates! Automatic update may (and most likely will) rewrite your customizaton, so you will need to update this after each change. Therefore it might be more effective to edit the INVOICE constant only withou modyfing the code in invoicehtml.php.