Currently I am working on user sign up feature and I am struggling a little finding the best way to validate that country input field only contains valid values and the same for city input field.
Once you have the reference data set/source, building the validation on a field or using a select (like live_select) should be pretty straightforward …
Hope this helps.
ex_cldr_html provides a CLDR-basedterritory select dropdown (dead view version). It might give you some ideas for a live view version (CLDR-based components is “on the list” with no committed timeframe).