Dual booting guide:

http://library.morph.zone/Dual-boot_MorphOS_and_MacOS_X_on_a_Mac_Mini_G4

I would not recommend even trying triple booting on a 40GB drive, the limited space would be better suited to a single or dual boot setup.

And I think you can change the default languages used under OSX (at least with the copies they sell us in the US).
As to upgrading to 10.5, I don't even bother with that.
If I need 10.5 on a Mac, I just buy a drive with that pre-installed.
If you add a second drive to your system for MorphOS, you will only need a small partition on the OSX drive to start booting MorphOS.