FYI,

I have succesfully modified a V25 CD to support the dutch language on a MKIII.
With help from the programs by Jochem (NavTweak/Mode2Cdmaker).
I have copied the nl_f.csf file from the MK4 folder and copied this to the MK3 language folder. Renamed the file to gb_f.csf, modified all text strings using NavTweak in the usw_load file.
Then renamed the 0501 folder to 0502 and modified the sw file and lng files to force the Mk3 to load the updated languages.
If I select GB in the settings menu then everything goes in Dutch, including the spoken directions.

Next thing I'm gonna try is to code the MK3 into actual Dutch in stead of a 'hacked' English.