define ('WPLANG', '');

to

define ('WPLANG', 'es_ES');

if your language filename is es_ES.mo I suggest making this change before you install WordPress.

This is all well and good, but this post is more about dispelling some of the myth and confusion surrounding .mo files. .mo files are not human readable by their nature and cannot be edited directly. You would need to install a version of poedit on your computer to be able to modify your language files and create new ones. There is detailed information on the Codex on where to start and what to do.

BUT .mo files can be decompiled into their .po counterparts which can then be edited using poedit. To convert .mo files into .po files, just cd to the bin directory of your poedit installation (in windows you have to use the command prompt) and use the following command

msgunfmt [path_to_file.mo] > [path_to_file.po]

Now you should be able to edit the po file created with poedit. Follow the Codex instructions at that point. You should not have to depend on anyone for the source of their .mo files if you need or want to make a change.