It's getting better each year though. On a trip to Italy I bought NightJet and EuroCity tickets via ÖBB's website, and a Frecciargento ticket via Trenitalia's website. All from the comfort of my Dutch home. Tickets for local trains can be trivially bought at terminals which speak a host of languages these days at most Italian stations.
At such long distances I prefer to plan a day or so at intermediate points (like Paris in your example) to prevent delays from cascading into the whole trip. So you would have separate tickets anyway.
I realise someone who speaks only Dutch and a bit of English may be daunted by that, but there are local train travel agents who can do it for you for a modest fee.