As reported by Vladimir SEKELJ in http://moodle.org/mod/forum/discuss.php?d=162784
I just tried selecting a student from the participants list then sending them a message. The email copy was as follows:
<p><a href="http://lang.moodle.org/">Moodle languages portal</a></p>
I guess the reason for this issue is that when a teacher send a message to selected participants the html editor is available.