The answer is "it depends". Note that either method described below will delete all of your apps/text messages/phone call log/etc. You can certainly back them up and restore them (SMS backup/restore is free in the market). Also be sure you have all of your contacts backed up in Google so that you don't lose anything. Then it's on to resetting the phone:
OPTION 1:
If you have installed recovery on your phone (ClockworkMod) then you can boot into recovery and just wipe data to put the phone back as if you had just installed Android. So you can try this:
1. power down the phone
2. press and hold the power button until it loads into the black/white screen with the MAGLDR menu
3. Use the volume up/down buttons to navigate and select (I think) option 8 to enter into "AD Recovery"
If recovery is installed then it should load up. From here you can again navigate with volume up/down and you'll want to:
- wipe data
- wipe cache
- advanced > wipe dalvik cache
Now exit back to the main menu and select the option to reboot your phone. It should take longer than usual, but bring you to the setup screen where you can choose your language.
OPTION 2:
Download another ROM image (or the same one you have currently installed) and boot the phone into MAGLDR by holding the power button at startup. Select the "USB flasher" option (I think it's option 4) and then connect via USB to your computer and flash Android on there again. It is essentially overwriting your data/cache with new data so this is pretty much the same thing as wiping it via recovery. Again, once the phone reboots it should run through the first time setup and one of the steps there should be to choose your language.
