understanding difference between changing language of theme vs. dashboard

Please help me understand this:

1. I have been able to install the Spanish back end version for WP.
2. I would like to be able to also change the language of this theme into Spanish (the front end)
3. There is one instruction that is confusing: instructions say that the language code in the wp-config.php should be changed. I have already done this to change the WP language to Spanish in #1. How do I change the language of the translated theme in #2? These two things seem to be the same process. But how do I change the theme?
4. When translating a theme i see that a few themes say they are translation ready and offer a .pot file. The theme I am interested does not have a .pot file but has a en_EN.po and en_EN.mo files. Which one do I need to translate in order to translate the theme into Spanish?

Thanks for your help