As far as I know, that's expected behaviour. All non-English language packs (including English variants such as "en_us") are being searched in location specified as $CFG->langotherroot which defaults to $CFG->dataroot.'/lang' - see this code in lib/setup.php around lines 211-214:
// Location of all languages except core English pack.
if (!isset($CFG->langotherroot)) {
$CFG->langotherroot = $CFG->dataroot.'/lang';
}
If for some reason you need to have these packs in non-standard location, you should be able to define $CFG->langotherroot directly in your config.php.