This is the language which dominates because there are many countries which use the same language. That's why, you need to specify to language first and then precise with the country.
Thus, the most appropriate is
However, there is no impact on SEO, it's just a standard.
For this kind of stuff, you can just take a look how big companies do. For instance, Facebook has URLs like:
https://fr-ca.facebook.com (for Canadians who speak French)
https://fr-fr.facebook.com (for Frenchmen and Frenchwomen who speak French)
https://pt-br.facebook.com (for Brazilians who speak Portuguese)
https://pt-pt.facebook.com (for Portuguese men and women who speak Portuguese)