It’s a great question :-). The ones in Paris didn’t say actual words to me, they were just weeping. The girls in Portugal spoke in a language I didn’t recognize (it wasn’t modern Portuguese), but I just understood them.
In Buenos Aires the previous year, the ghost I found spoke an oddly inflected Spanish, which I understood, but I do speak Spanish. And in Scotland, they spoke in what I assume was some earlier form of Gaelic, but again, it was just clear to me what they were saying.
All the others have been here in the States and spoke in English.