After the latest update it seems you’r not asked if you want to save in mm when saving to DXF.
Why is this feature removed?
In the latest verion of DELFTship you set the project units in the Project settings:
On the Coordinate tab you can set metric / imperial / custom.
In the details window, general information section you can set dimensions to meters, mm, ft or inches.
This setting is now used when exporting.