Rescue data from Powerbook G4 with forgotten password
As a first step, I would try ejecting from the Optical drive again. However, this time try this:


Ensure the PowerBook is switched off
Press the mouse button (or trackpad button) down and keep it down
Restart the PowerBook and keep the button down until something ejects or you get the same language prompt again


Assuming this ejects a CD/DVD, then try the Install disk option.

NOTE: If these steps don't work, you may want to try again with a USB mouse if you didn't already (just in case there's an issue with the trackpad).

[EDIT]

If you can't use the Optical drive, then you have two options. 


Purchase a FireWire cable (as you've already noted) and use Target Disk Mode
Remove the hard drive altogether and use it in an external case/enclosure


Of course, option 2 will depend on whether the intention was to still use the PowerBook and whether or not you feel comfortable with the hard drive removal.

If you'd like to determine your willingness to do this, here are the instructions for doing so with that PowerBook (based on your description).

One advantage of this approach (besides the fact you can pick up external cases for next to nothing) is that you can potentially still use the hard drive as an external drive once you've recovered the data. You can also then choose to reformat and reinstall the system and replace it back into the PowerBook (if there is still a purpose for it), or place another HD into the PowerBook.