In include/requirements.inc.php, the mbstring check is actually doing the same thing as the bcmath check:

function check_php_mbstring(){

$current = function_exists('bcadd') &&
function_exists('bccomp') &&
function_exists('bcdiv') &&
function_exists('bcmod') &&
function_exists('bcmul') &&
function_exists('bcpow') &&
function_exists('bcpowmod') &&
function_exists('bcscale') &&
function_exists('bcsqrt') &&
function_exists('bcsub');

This should be checking for mbstring functions instead.