Installing new firmware: PDA,CSC
PDA/CSC explained in one of my previous answers here. As for your situation, there are 2 factors to consider:


Generally, the release date and version of what you're going to flash should be equal to or newer than what you had on the phone right now, so that it will flash at all. Downgrading is not entirely impossible - on newer devices, bootloader level determines how far you can go back; see part of another answer here.
If you need certain languages to be present, look into the CSC string and make sure it's from your general region.