Apple Watch Series 5

If it's a GPS only version of the watch then yes it will work in those countries.

If it's a cellular version you would have to contact the network providers in those countries to see if they support the apple watch.

Yes just remember that whatever network provider you choose it must be the same one for both phone and watch.Also a network carrier you choose say in Australia may not work in South Africa,each country has different and usually incompatible network providers.

I would suggest to you to buy the GPS only version of the watch,this will work in all countries,the only proviso is that you have to have your phone with you to make calls and send texts.

You have to contract with each carrier to add the Apple Watch to your iPhone plan. Seems like a lot of work for a visit, unless you visit for a few months at a time.

I think it would just be easier when you are visiting outside of Spain, that you just treat your Apple Watch as a GPS only model, and it will do everything the cellular model will do as long as you are near your iPhone. I suspect you will survive for a visit keeping your iPhone nearby.