You mention that other fields are validated but not the region field, that your primary motivation is for mailing addresses. If this helps you at all, only one of those countries you mention in your list of examples (USA) uses the region name in the mailing address.
I took an entirely unscientific examination of a random set of industrialised or industrialising countries and broken them into three sets: require a sub-unit in addresses, may require a sub-unit name (long-form), and do not require a sub-unit. I gathered the information from the UN agency responsible for postal services, and include the English name or names of the countries sub-units, where I know them off-hand.
Nations that require a sub-unit abbreviation in their postal addresses
- United States: state, though in reality also districts (D.C.) and territories (P.R., Guam et al.)
- Brazil: state
- Canada: province/territory
- Australia: state/territory
- Mexico: state, though in reality also federal district (D.F.)
- Italy: province
- Venezuela: state
Nations that include sub-national units, and not always required (e.g. a large city may do without, but a rural village will need it)
- India: state
- China: province
- Korea: province
- Japan: prefecture
- Russia: province
- Spain: province (there are also communities, but it looks like provinces are what are used for the mail)
- Switzerland: canton (abbreviation is optional only when the town name is not unique. Perhaps a Swiss counterpart on this forum can speak to user behavior here; do Swiss nationals just add the abbreviation after the town in the town field?)
- Thailand: province
- Ukraine: province
Nations that do not use subnational units in their postal addresses
- UK: county/nation/unitary authority (?! from what I can gather, it's a mess)
- France: department
- Germany: federal state
- The Netherlands: province
- Belgium: region or province
- Ireland: county
- New Zealand
- Argentina
- Algeria
- Israel
- South Africa: province
- Poland
- Austria: federal state
Conclusions
- Many countries do not need sub-unit information in their postal addresses
- The majority of countries that do call their sub-units state or province when translated into English
Again, this is an unscientific sampling, but I think it gives us enough information to say that you are probably okay to use the "State/Province" convention in your form, and possibly hide the field except for countries that you know you need information for it.