How to brick-proof my phone before flashing to another language

[roy_hu]

I got my Diamond from Sprint. I'm planning to flash it with a ROM in a different language, and restore the phone later. I have some concerns before I pull the trigger.

1. I only want to change the OS so that all the interfaces can be understood by someone that knows no English. I want to keep the Radio intact. Can I tell if the ROM only contains OS and that the Radio will not be overwritten?
   
2. I'm not sure if the Rom I downloaded is for GSM or for CDMA. Most likely it's for GSM. Will I be screwed if I flash the Rom? If it's just that the programs cannot use the phone and I can restore it with, say, Juicy's rom, then I'm not worried.
   
3. If the entire stack is overwritten, can I go back? What's the backup/restore plan?

Thanks!