$arr = array(
    'parent' => 5
);

$letters = range(chr(0xC0), chr(0xDF));

foreach ($letters as $b )
{
    $abc[] = iconv('CP1251', 'UTF-8', $b);
}
foreach ($abc as $cat_name ){
    wp_insert_term(
        $cat_name,
        'category',
        $arr
    );
}