Help with WordPress localization

Hi everyone!

I am building a site of WP and facing this situation:

– Frontend language should be in Vietnamese
– Backend language should be in English

I use WordPress 4.3

In wp-config.php, I set $locale = 'vi';

Then, in theme’s functions.php, I use the following code:

function setAdminLang($locale) {
    if( is_admin() ) {
        $locale = 'en_US';
    }
    return $locale;
}
add_filter('locale', 'setAdminLang');

The frontend displays Vietnamese OK.
But I need Admin to display English, and now, it still displays Vietnamese.

Please help if there is something wrong here.

I learnt that $locale is a new way to set language for WordPress 4.0 and above. But maybe there is still something buggy?, I don’t know.

Thanks so much!