Set the location of the language directory.

To set directory manually, define the `WP_LANG_DIR` constant
in wp-config.php.

If the language directory exists within `WP_CONTENT_DIR`, it
is used. Otherwise the language directory is assumed to live
in `WPINC`.

function wp_set_lang_dir() {
	if ( ! defined( 'WP_LANG_DIR' ) ) {
		if ( file_exists( WP_CONTENT_DIR . '/languages' ) && @is_dir( WP_CONTENT_DIR . '/languages' ) || ! @is_dir( ABSPATH . WPINC . '/languages' ) ) {
			/**
			 * Server path of the language directory.
			 *
			 * No leading slash, no trailing slash, full path, not relative to ABSPATH
			 *
			 * @since 2.1.0
			 */
			define( 'WP_LANG_DIR', WP_CONTENT_DIR . '/languages' );
			if ( ! defined( 'LANGDIR' ) ) {
				// Old static relative path maintained for limited backward compatibility - won't work in some cases.
				define( 'LANGDIR', 'wp-content/languages' );
			}
		} else {
			/**
			 * Server path of the language directory.
			 *
			 * No leading slash, no trailing slash, full path, not relative to `ABSPATH`.
			 *
			 * @since 2.1.0
			 */
			define( 'WP_LANG_DIR', ABSPATH . WPINC . '/languages' );
			if ( ! defined( 'LANGDIR' ) ) {
				// Old relative path maintained for backward compatibility.
				define( 'LANGDIR', WPINC . '/languages' );
			}
		}
	}
}