In 5.1 legacy stack, there is a call to htmlentities in kernel/content/treemenu.php (only one in the whole kernel)
$childResponse['name'] = htmlentities( $child->getName() );
But, according to the PHP documentation, before php 5.4 htmlentities is using ISO-8859-1 encoding and UTF-8 starting from php 5.4. Our requirement is php 5.3.
My issue here is that Japanese node names get turned into 'null' because of this call.