create or replace function custom_translate(p_string varchar2, p_default varchar2, p_xz varchar2) return varchar2
is
  l_return varchar2(4000);
begin

  with
    t1(fs, rn)  as (
                select
                  substr(p_string, rownum, 1),
                  rownum
                from dual
                connect by rownum <= length(p_string)
               ),
    t2(fsv) as ( select
                  substr(tst, rownum, 1)
                from (select p_default tst from dual)
                connect by rownum <= length(p_default))
    select listagg(nvl(fsv, p_xz), '') within group (order by t1.rn) ret
    into l_return
    from t1 left join t2 on t1.fs = t2.fsv;
    
  return l_return;
end;
/

select custom_translate('Говнокодер-нехорошийчеловек', 'од', '?') xz from dual;