updated [SK] translations

The documentation from BaseYii.php @line 455 would create extra translation file "app.php" with a single line:

     * echo \Yii::t('app', 'Hello, {username}!', ['username' => $username]);

Also I18n (plural/singular) may need a little examples on how to translate properly, because sometime it is difficult to guess correct declination - especially for those who did not worked with formaters yet.