SNCF has an app (SNCF connect) where you can buy Pass Navigo Decouverte semaine. (Il de france tickets on your phone. Digital tickets.) I thougt, only french people could use the app but the app is available in Germany and you can read all instructions in german. Do you know if it´s possible to buy it as a tourist?
Thanks for your anwer
The SNCF Connect app is open to anyone to use, not just France residents. The important caveat is that not all phones are supported. Only some Android phones can store tickets and Navigo pass subcriptions in the SNCF Connect app. And no iPhones are supported currently.
So if you're using an Android phone and the app allows you to purchase individual tickets or Navigo subscriptions, then yes it's fine to use regardless of residency. The app will perform a technology check prior to purchase to ensure the phone has the necessary hardware/software requirements for use.