Hi Jane,
Mostly, if your newer Mac came bundled with a certain OS, in this case your iMac G5 came with a Tiger disk, the OS disk is specific to only certain Macs and usually more specific to that Mac. That's just how Apple designed the disk.
If it doesn't install, and it will tell you early in the process, you will have to get a retail version of Tiger.
You don't need to install another OS first and Delocalizer will remove the languages. But, before installing, while being prompted, go to options and deselect extra languages and any printer drivers that you don't need. Then you won't have to run anything to remove those things in the first place.
Have fun.