If you also want to try it out, be sure to check WPML compatibility. Coming from Polyglot myself I’ve written a very Q&D convertor which only creates output for a SQL loadfile, as always, use at your own risk.
PHP code for polyglot_convert.php
$database="yourdb";
$dbuser="youruser";
$dbpassword="yourpw";
# We need 2 functions from WordPress' wp-includes/formatting
include("../wordpress/wp-includes/formatting.php");
mysql_connect("localhost",$dbuser,$dbpassword); $query='select * from wp_posts where (post_title like "%lang_nl%" OR post_content like "%lang_nl%") AND post_status="publish" AND post_type="post" ';
$result=mysql_query($query);
$i=0;
while ($i < mysql_numrows($result)) {
$title_nl=addslashes(preg_replace("/\[.?lang_nl\]/","",preg_replace("/\[lang_en\].*\[\/lang_en\]/","",mysql_result($result,$i,'post_title'))));
$title_en=addslashes(preg_replace("/\[.?lang_en\]/","",preg_replace("/\[lang_nl\].*\[\/lang_nl\]/","",mysql_result($result,$i,'post_title')))); $title_en_short=$title_en;
if ($title_nl == $title_en ) { $title_en_short=$title_en_short."2"; }
$content_nl=mysql_result($result,$i,'post_content');
$content_nl=preg_replace("/\[.?lang_nl\]/","",$content_nl);
$content_nl=preg_replace("/\[lang_en\].*?\[\/lang_en\]/sU","",$content_nl);
$content_nl=addslashes($content_nl);
$content_en=mysql_result($result,$i,'post_content');
$content_en=preg_replace("/\[.?lang_en\]/","",$content_en);
$content_en=preg_replace("/\[lang_nl\].*?\[\/lang_nl\]/sU","",$content_en);
$content_en=addslashes($content_en);
echo "UPDATE wp_posts SET post_title = '".$title_nl."', post_content = '".$content_nl."' WHERE ID=".mysql_result($result,$i,'ID')."; ","\n";
echo "INSERT INTO wp_posts SET post_title = '".$title_en."', post_content = '".$content_en."', post_date = '".mysql_result($result,$i,'post_date')."', post_date_gmt = '".mysql_result($result,$i,'post_date_gmt')."', post_author = ''.mysql_result($result,$i,'po
st_author')."', post_category = '".mysql_result($result,$i,'post_category')."', post_name = '".sanitize_title_with_dashes($title_en_short)."'; ","\n";
echo "insert into wp_icl_translations set element_type='post_post', element_id=LAST_INSERT_ID(), trid=".mysql_result($result,$i,'ID').", language_code='en', source_language_code='nl';","\n";
$i++;
}