So your theme takes only the spanish menu and not the eureska menu.
Make a test to be sure:
Deactivate polylang and try with your theme to assign the last menu that you have created, for you eureska. If your theme shows at each time spanish menu it means that is an incorrect usage about wp_nav_menu by your theme.
Did you alerted your theme author about this issue, he looked further on register_nav_menu and wp_nav_menu?
http://polylang.wordpress.com/2014/04/10/incorrect-usage-of-wp_nav_menu/