In OFBiz go to Facility -> Contact Information -> New Contact Information
Select "Postal Address" in the dropdown and click "create". In this form the "State/Province" dropdown is dependant on the "Country" dropdown. Once a country is selected the States of that country will be in the selection of the first dropdown. This behaviour should be part of every form which has an address as an input. The dropdown dependencies are realized with the getDependentDropdownValues function in miscAjaxFunctions.js.
A place where this functionality is missing is in the HR section of OFBiz. Got to HR -> Employees -> New Employee (https://localhost:8443/humanres/control/NewEmployee)
Here, the "State" dropdown lists ALL states which are listed in the geo entity. It would be nice to implement dependant dropdowns here aswell.