You can just create a copy of the English language files (/administrator/components/com_nbill/language/en-GB/) in a folder named according to the language you are using (eg. fr-FR, en-US, pt-BR etc.), rename each file, again to replace the language name, then replace the English text in those files with text in your own language. If you would like to donate the translation as a language pack that can be used by others, just send the files to me and I will create a language pack for it.