The error happens in
application/helpers/admin/label_helper.php (53-55):

if (isset($sqlvalues))
foreach ($sqlvalues as $sqlvalue)
Label::model()->insert($sqlvalue);

According to this link: http://www.yiiframework.com/forum/index.php/topic/8900-how-to-insert-data-record-using-insert-method/

insert() is a lowlevel function and one should rather use save() in stead of insert().

I have substituted this code for the three lines above.
It creates a new Label object for each new label and sets each sqlvalue property individually, then saves to database.

if (isset($sqlvalues)) {
foreach ($sqlvalues as $sqlvalue) {
$label = new Label();
foreach($sqlvalue as $k=>$v) {
$label->setAttribute($k,$v);
}
$label->save();
}
}

I have not tested extensively but it appears to work: no errors languages are added as expected, labels for one language do not disappear when other languages are updated.