I've used monolingual on a number of systems. It's a one pass program, that if you don't choose your options carefully, can get you into trouble. However, you chose some version of the Queen's English, so that should be OK.
What's not clear is .... what disks are you talking about? Are you swapping in hard drives? external hard drives? CD/DVDs? or ? Do you have a bootable external hard drive? Or the original Tiger 10.4 DVD's? If you boot the system holding the alt/option key down, and choose to boot from DVD or an external hard drive, I can't imagine what could cause anything to erase automatically. You'll need to step through what you're doing, because this is a very odd situation.
You can probably run Leopard 10.5 on your PowerBook, which is the operating system after Tiger 10.4, unless you need the OS X classic environment, but that's not your issue today.