How to use a language file to edit plugin output text?

Pretty much what the title says.
I'm trying to edit the "Buy Now >>" text shown with the Global Product List shortcode in Marketpress. I want it to be "Buy Now" (w/o the >>:wink:.

This is a side thread from:

I originally edited the marketpress-ms.php file to get rid of the arrows, but this will be overwritten on an update. @DavidM suggested a better way through a language file.

Let's see how to.

Can someone help with this?