Added multi-language site tag to add method and xml view.

I have fixed this issue by adding translation array the same way you handled images.

https://github.com/RoumenDamianoff/laravel-sitemap/issues/13

```
// elements must be in nested array with 'url' and 'language' keys
$translations= array(
                 array(
                      'url'=>'http://domain.com/en/topic',
                      'language''=>'en'
                   ),
                  array(
                      'url'=>'http://domain.com/topic',
                      'language''=>'hr'
                   )
                   // etc.
                );

$sitemap->add('http://domain.com/topic', $modified, $priority, $freq, null, null, );
```