Language packs can be installed using the nBill Extensions installer. They are compatible with both Joomla! and Wordpress. Language packs usually lag behind new releases somewhat, so even with a language pack installed, there might still be some English text. You can translate this yourself though (please also see the FAQs about translating nBill). Some language packs only work on the text in the front end - please check the notes for each language pack by clicking on the 'Details' button.