If you visit other course from navigation menu, apache's error_log file will record something like "json_encode(): Invalid UTF-8 sequence in argument". It seems json_encode() function received none utf8 character. Navigation menu can't work sometimes. I found some problematic code in lib/navigationlib.php line 4000:

if ($child->forcetitle || $child->title !== $child->text) {

    $attributes['title'] = htmlentities($child->title);

}

htmlentities use ISO-8859-1 character set as default, but my course title has Chinese character, after called htmlentities, these Chinese character will not be recognizable any more. I think the code should be changed like this:

if ($child->forcetitle || $child->title !== $child->text) {

    $attributes['title'] = htmlentities($child->title,null,'UTF-8');

}

I found moodle called htmlentities in many places, if these callings use ISO-8859-1 as default charset, the Asia user of moodle will suffer inconvenience. I hope all these callings will be checked carefully.