Hi thomasdk81,
I had the same problem yesterday and today I’ve found the solution: as you are using a “mu-plugin”, you shoud not use load_plugin_textdomain but load_muplugin_textdomain
From wp-includes/I18n.php :
/**
* Load the translated strings for a plugin residing in the mu-plugins dir.
*
* @since 3.0.0
*
* @param string $domain Unique identifier for retrieving translated strings
* @param strings $mu_plugin_rel_path Relative to WPMU_PLUGIN_DIR directory in which
* the MO file resides. Defaults is empty string.
*/
function load_muplugin_textdomain( $domain, $mu_plugin_rel_path = '' ) {
$locale = apply_filters( 'plugin_locale', get_locale(), $domain );
$path = WPMU_PLUGIN_DIR . '/' . ltrim( $mu_plugin_rel_path, '/' );
load_textdomain( $domain, trailingslashit( $path ) . "$domain-$locale.mo" );
}
so:
load_muplugin_textdomain( 'wordpress-mu-domain-mapping', 'languages');
should work.