Sorry to resurrect this thread but I have the same problem. Nexus S with 1.5.13 ROM + English language pack downloaded from the main page. I wipe everything (cache, data, dalvik) then install the rom. I have then tried installing the lang pack straightaway from CWM, that sticks on the Google screen. I then wiped dalvik, cache then installed the lang pack, that also sticks on the Google screen.
Rebooting into MIUI without installing the lang pack is fine, although in Chinese. I then tried d/l Rom Manager and installing the lang pack from there, wipe cache, wipe dalvik. That sticks on the Google screen also.
I am sure there is something I have missed. I have d/l'd the lang pack a couple of times just to make sure it is coming down ok.
Thanks for any help and I thank you for all your hard work that makes MIUI the premier Rom for the Nexus S right now.