Why don't you do something more clever?
As most (about 90%) of services when you identify you get "+r" usermode, you can start auto-join if you receive that usermode flag instead of NOTICE from nickserv that if i use an other services language non-english it may be broken.
Raccoon pointed out that there are several different usermode +r alternatives, so mIRC would in that case have to cater for multiple different representations of usermode +r based on server version numbers.
