Hi,
This all depends on how you have added the
es/theme_snap.php
I you have added it to the Snap theme's lang folder so that it look like this...
snap/
lang/
en/theme_snap.php
es/theme_snap.php
Then you need to go to Site Administration > Language > Language Packs
and update the first column where is says something like
Update all installed packs
After this you should see the changes to the theme.
Hope this helps?
Mary