I have installed Core 5.3.0 Everything is in metric units, except for the offset measurements in the imposition layout screen (in this case I have copied an imposition preset and I want to make changes to it, but I can't switch to metric units).
All possible region or locale infor is set to the metric system.
My guess is you have chosen a press sheet size that is designated in inches. This results in the offset measurements being in inches. If the press sheet is designated in mm, mm should show up in the offset measurements.