Does the vw g12 coolant can be used undiluted?

No, you cannot use undiluted G12. It will gel up and you'll have serious issues.

However some dealers sell a pre-mix version and some sell straight coolant. You need to check your bottle carefully to see if it's pre-mix. The pre-mix should say so, while the straight coolant will say nothing about it. It is safer to double-mix than to run over-rich coolant.

And ALWAYS use only distilled water to top off or mix!