Wordpress - How to change language in Hosting service

It's just really weird that when I deploy my wordpress site on to the hosting service, the language of the site is still English (the default), and the qtranslate plugin did not work. Thing is everything is just OK in my laptop. What's wrong with the host?

And, I figure out that I have to define the language directory of my wordpress site, and everything is OK:

define('WPLANG', 'vi');
define('WP_LANG_DIR', $_SERVER['DOCUMENT_ROOT'].'/wp-content/languages');