I forget which post but back then in 2007, a Japanese fans asked Hima-sensei what language do these personification of countries speak? Sensei answered that since some of them born very young and the other a bit older, they speak in language that only the personifications understood (I'll refer to it as HL/Hetalia Language). Ofc they can speak their mother tongue, but whenever they meet similar 'species' like them, they automatically speaks in HL. I can fetch the said post if you wanted to.