where to install the mo files for spanish for a sub blog

Ok. So I have a multisite WP installation. I was looking at the file structure and am not sure where to install the es_ES.mo file to provide the Spanish version.

I would like to choose which blog to add the spanish version to.
I am attaching a screenshot of my file structure...
there I found the wp-content folder, is it there where i would place the mo file?

Also is it in that folder where I would find the file wp-config.php to modify it? Is this going to change the language of the whole network?

Thanks for your help