Well, I was able to find the Language setting, and the drop down box didn't have any text, although there was one invisible item, so I'm sure that was English. I added all the rest of the available languages through the avast setup, and all of the languages had the same basic problem, and none of them were displaying properly. Some words did, but most of the UI was still messed up. I tested that without rebooting, though... but it did switch the languages on the fly, and like I said... so far, no good.
I also booted Windows in safe mode and avast still looks messed up, so we can pretty much rule out anything that is disabled in safe mode.
After all the troubleshooting I've been doing with my computer, I've noticed it's starting to respond more slowly than usual. Uninstalling/reinstalling drivers, frequent reboots, etc... I don't know what's wrong. Blegh!