.htaccess RewriteRule to preserve GET URL parameters
Solution 1:
You need to append with the [QSA] (query string append) tag. Try
RewriteEngine on
RewriteRule ^([a-z]{2,2})/([a-zA-Z0-9_-]+)$ index.php?lang=$1&page=$2 [QSA]
See http://httpd.apache.org/docs/2.2/mod/mod_rewrite.html