This is a very little bug.

in admin/roles/assign.php

$strpotentialusers = get_string('potentialusers', 'role');
$strexistingusers = get_string('existingusers', 'role');
$straction = get_string('assignroles', 'role');
$strroletoassign = get_string('roletoassign', 'role');
$strsearch = get_string('search');
$strshowall = get_string('showall'); <- Here!
$strparticipants = get_string('participants');
$strsearchresults = get_string('searchresults');

Please change
$strshowall = get_string('showall');
to
$strshowall = get_string('showallusers');
since get_string('showall') needs a parameter.

There are no influences in english environment([Show all]), but a strange string is displayed in Japanese environemt .