I have never seen a Kubota diesel that is 6 volt. I would bet that it is 12 volt. If you still question this look around on all the electricals and see what their ID tag says.
Everything is in japanese, even the battery doesn't have anything on it. I was thinking it would be 12v but wanted to make sure in case I ever need to boost or charge the battery.
As Motorhead says I also think it is 12 volt. Can you tell how many cells the battery has? (how many places to add water) Even a sealed maintenance free battery you can usually count the cells. If it has 3 cells it is a 6 volt battery. If it has 6 cells it is a 12 volt.