wp-invoice change language

Hi I have a problem I need to translate the texts are in English to another language. I can not find the file where it is displayed. I have attached an image capture where I marked in red text that I want to change the language.

And also try to find the months of the year to modify the language to ask for download in pdf.

Can you help?

View image: http://www.seogirona.es/images/invoice.png

http://wordpress.org/extend/plugins/wp-invoice/