How does buying DLC with a dic game work?

(Discussion flair cuz there ain't no question flair) I know that I need to buy a dlc of the same region as my disc's origin but for example if I know my disc is a European disc but don't know what country in specific can I just use my French account to buy dlc or do I have to find out what country exactly and buy DLC from an acc of that country if so how do I find out what country my disc came from?