Slow work without i18n

[floor12]

I noticed what if the app does not use i18n at all and app has not any lang files, method ::getLabel() work really really slow, because it tries to find get label with Yii::t(). I noticed this problem in gridView.
I think the right solution is to change
public static $messageCategory = 'app';
to
public static $messageCategory;
and make a check in ::getLabel() method.

I noticed, if i have disabled i18n my yii2 app, much faster use Enum::$list[$id] instead if Enum::getLabel($id). The difference is hundreds of ms.