In the wordpress wp-config file I have the following.
* Language of WordPress.
* Change the following to have WordPress in your language. The corresponding MO file
* The chosen language must be in wp-content/languages.
* For example, to install ca_ES.mo wp-content/languages copying and WPLANG defined as 'ca_ES'
* Catalan dictionary for WordPress.
define ('WPLANG', 'es_ES');
My manager wordpress is all in Spanish, but the plugin does, is still English.
Not have a tutorial or something where I can look and see what the errors, that just rename my files just es_ES.po es_ES.mo and that is how they are all my wordpress administrator and or that way you change language plugin.