Localization is used to simply set the weight units to be used in your Prestashop store.
Go to BackOffice>>Preferences>>Localization

In the weight field enter the weight unit you wish to use and click Save
The weight unti is used on your rpdouct weights and in your shipping weights.
Be aware that you cannoy enter just any code. For example for kilogrammes enter Kg and not kilogrammes as Prestashop will reject this. Although you can enter K or Kilo for kilos.
We also need to be aware of the units being used and how Presatshop calculates these as there can be issues with calulating shipping costs depending on what has been used (more info on this when I have it available but ibs and kilos are ok)