Operating System discs carry several folders, some of them specific to particular navigation units series.

Modifying the disc for dutch probably means fooling the system by replacing a file with English text strings or sound data with a similar file with the corresponding sound or text translation. The system will read these files and display whatever strings or reproduce whatever sounds they may contain, assuming it is English and never knowing it is actually dutch what it is displaying or reproducing.

Now this file replacing has to be performed in the Mk-IV specific folders of the CD if you want to fool a Mk-IV system. And in the MK-III specific folders, if your target is a MK-III. Or in each and every specific folder, if you want your modified CD to be universal.

You should investigate if the modificatios of the system CD relate to Mk-III system. If that is not the case it makes no sense trying to upgrade a Mk-III with it.

Regards