Airline tickets on most (all?) airlines these days are priced in the (usual) currency of the country of origin of the ticket, that is, where the first airport you depart from is located. (well, we could of course get into an arcane technical discussion about IATA NUCs and how airline fares in the IATA system are constructed but that is probably not so useful to your question!)
So in your case, all ex-USA tickets will be priced in USD.