I had to replace a bad speedo cluster;
for the replacement, i had to tranfer the mileage from the old to the new ( i had the odometer returned to zero first; by reflect tuning). During yhat process, in the soft coding in VCDS; you are to change the display from miles to kilometers or visa versa. Keep in mind, this was a North America/USA spec speedo cluster; i don't know, abiut parts from countries that are natively kilometers vs miles. Keep in mind you will probably need the secret key code; to be able to change the soft coding (this requires a cable/software like vagtacho) or have rhe changes doen by a specialized shop like Reflect Tuning.
To answer your question, a kilometer speedo; will most likely be programmed to display km's.
The other question would be, what is wrong with your original cluster and what do you want in your repair/swap results? The original speedo, maybe repairable; from places, like bbareman with a lifetime warranty. Let us kniw, what you want to ultimately with your current situation, thanks.
I guess it depends on how much time, money and effort; you want to spend on changing it over, to get km's to be displayed. Unfortunately, the older cars; make it harder to change the display and requires soft coding changes in the cluster.
A cheap and easy option; could be a obd II dongle like the ELM 327 (wifi for ios/apple; bluetooth android os) and setup a app on your phone (dash command, etc), to display your speed in km's and you can see other things as well (coolant temps, etc).