I was translating a WordPress plugin to Dutch, and was ready to test it. So I visited the test site’s settings page where you can change the language. Sure enough the UI was now in Dutch, but the plugin text wasn’t. Everything seemed to be in place: files in the right folder, files had the correct filenames, load_plugin_textdomain was set to run during the plugins_loaded action, but it just didn’t seem to accept the files.

The last missing ingredient appeared to be wp-config. Even though WordPress recently allows you to change the language right from the settings page, it still looks at the WPLANG constant defined in wp-config.php to overwrite that preference for themes/plugins (even if the constant’s value is empty). So be sure to go into your wp-config.php and comment out the WPLANG constant to make WP ignore it.

// define('WPLANG', '');