'pro-national' is a compound formed from the prefix 'pro-' (from Latin 'pro', meaning 'for') and 'national' (from French 'national', ultimately from Latin 'natio' meaning 'birth, tribe, nation').

'national' entered English via Middle French 'national' from Latin 'natio'; the productive prefix 'pro-' (Latin 'pro') has long been used in English compounds (e.g. 'pro-democracy'), and 'pro-national' follows that model.

Originally the elements meant 'for' + 'nation' and the compound straightforwardly meant 'in favor of the nation'; over time it has retained that sense but is now often used specifically in political contexts to mean support for nationalist policies or state-centered approaches.