Change text "Add to cart"

Hello
I'm spanish and need change "Add to cart" text in button, but I can not find the file for editing.
You can see the store demo mode http://www.tarjetasdepuntos.com/tienda_online/
Thanks for your help

Hi,

I went to your site but didn't see the text, you seem have translated it already. Anyways, you can find such a text in \language\en-GB\en-GB.tpl_gk_yourshop.ini