Add Vietnamese language

[aoopvn]

I just add Vietnamese language for messages, so that anybody can get benefit from it.

Thank you very much for your awesome work!

[aoopvn]

Btw, just in case that other pull request for multi-language is still on the waiting list, my suggestion below is a temporary way to allow user override the messages without losing their translation after run composer update. It's only my opinion.

public function initI18N()
{
      if (!isset(Yii::$app->i18n->translations['kveditable']))
      {
         //current code logic
      } 
}

then in main.php user can get an overridable i18n for kveditable, just like awesome @kartik-v 's gridview

'i18n' => [
            'translations' => [               
                //override kveditable config, just like gridview
                'kveditable' => [
                    'class' => 'yii\i18n\PhpMessageSource',
                    'basePath' => '@common/messages',
                    //'forceTranslation' => true
                ],
            ],
]

Then you got your translation in messages.