The theme is translation ready, so your best bet is to just provide a de_DE translation files with 'ago' translated to 'vor'. Here's a good article on translating WordPress themes: http://www.appthemes.com/blog/how-to-translate-a-wordpress-theme/
If you do make a full German translation of Expound, don't hesitate to send it over, so I can include it in the next update.