Translating WordPress backend from English to Spanish

Somehow even though this should be a easy task I was unable to find out how can I switch language in WordPress to Spanish language when I have already installed solution in English. There just seemed to be too much info about it, mostly focusing on frontend. I assumed that it would be via Plugins but I could not find any suitable plugin for (if you know one please let me know). Also it seems that I cannot do this in user level (User James would have in English and user Juan in Spanish) it looks like language setting is a global setting. Probably you need WordPress-MU to do it in userlevel(?). Though, having languages in userlevel would be a nice feature as well as having ability to add new languages via plugins.
In any case. If you are struggling on the same issue as me, here is a short instructions how to switch WordPress from English to something else take a look  Manually Installing Language Files.

– Download .mo file for example Spanish language in here
– Drop it to /wp-content/languages/
– Modify wp-config.php
define (‘WPLANG’, ‘es_ES’);