Allow to set locale in URL::to()

URL::to($url, $https, $asset, $locale = true);
Now if $locale is string and in_array
Config::get('application.languages') we set user selected locale.

Ex.:
Config::set('application.language', 'en');
$key = 'ru';
URL::to(URI::current(), null, false, $key);

We have: http://yoursite.com/ru/...