Hi,
thanks for providing this plugin, I work with the english version of wordpress for years and sometimes I need to build dutch websites too.
You understand that just switching the language is a great time saver :)
You're using a cookie to store the language, why?
setcookie('pls_language', $pls_language, time()+60*60*24*30, COOKIEPATH, COOKIE_DOMAIN);
It would be much better to store this value in the wp_options table, this way you can setup the language by user... just a suggestion.
Thanks again!
http://wordpress.org/extend/plugins/plugins-language-switcher/