[Q] Adding languages to unbranded HTC radar

CAB-sender takes a backup (technically "restore point") using the same mechanism that Zune does. If you didn't back up your older restore points, they were overwritten.

I'd suggest just flashing a European stock ROM. Here's a nice list of the ROMs for Radar phones: http://xdaforums.com/wiki/HTC_Radar/Shipped_Roms. Make sure the device version matches (i.e. the TMoUS one probably won't work, though I don't know what hardware a Radar purchased by way of Japan will use).

Your questions, in order:
1. Yes. These are the ROMs which are distributed on retail phones, and should include the standard language set for Windows Phones (which I'm pretty certain includes Spanish for the European phones, although I've never checked).
2. Yes. Flashing a stock ROM will make the phone behave exactly as though you'd bought it with that ROM, and it should work with Zune like any other Windows phone (including being updatable).
3. I suggest going with the newest ROM on that list, because it has the same major version number for its bootloader (2) and same revision version number for its radio (03). This reduces the risk of it not flashing correctly for some reason. That ROM is also 7740, so it will need fewer updates after flashing.

Do keep your restore point handy, in case the flash fails for whatever reason, but it shouldn't be needed. You can try recovering older restore points using the "Previous Versions" feature of Windows (requires Volume Shadow Copy service to not be disabled, and System Restore to be enabled for you user-profile disk and set to backup user files as well as system files).

Flashing a European ROM really seems like the best bet, though. You'll need to start the phone into SPL (bootloader) mode, which I believe requires turning off the phone, pressing and holding the Volume Down button, and turning it on again. You should see a screen with bands of color and info about your SPL in yellow text. At the bottom will be black text saying "Serial". Plug the phone into the computer. It will install some drivers (automatically, on Vista or later; on XP you may need to do this manually) and then the text at the bottom of the phone's screen should change to "USB". At that point, you can run the ROM Update Utility (RUU) that should have come packaged with the ROM image.

I'm afraid you've reached the limits of my knowledge of such things. The only time I've manually flashed anything, the download came as a packaged RUU "installer" (basically just a self-unpacking archive, so far as I can tell) and included simple instructions for using it. If you tell me what file(s) your download contained, I could try to tell you how to lay them out like my phone has it, but I'm hesitant because I really don't know the intricacies of RUU.

I'm guessing you already checked the Radar forums? Alternatively, if you look at the threads for the other ROM downloads, at least some of them start with RUU and end with .EXE (which is how my download came) instead of starting with IMG and ending with .NBH (my download included some NBH files, though). Maybe grab one of the RUU bundles, run it, watch what files it extracts, and then replace the NBH file with the one you already downloaded? I can't promise this will work, but it's worth a shot.

any updates here ?