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/**
 * WordPress Localized Language, defaults to English.
 *
 * Change this to localize WordPress. A corresponding MO file for the chosen
 * language must be installed to wp-content/languages. For example, install
 * de_DE.mo to wp-content/languages and set WPLANG to 'de_DE' to enable German
 * language support.
 */
define('WPLANG', 'ES_es');

Create the folder languages in wp-content and put the file es_ES.mo y es_ES.po

<?php echo date_i18n('F'); ?>