As far as I know you can only use the single country pass in that country, so as Saarbrucken is still in Germany that will not work. You would need to get to a French station to use your pass.

There are some border towns that have dual country status and the planner will show both “stations”. You then buy a ticket to the “German” station and then use your pass from the other station in the planner.

I don’t know if there are any on the Ger/FR border, but I know Basel has a German and a Swiss “station”.

On that route you would need to buy your German ticket to Forbach.

Ah, ok, that makes sense. I am traveling from Frankfurt to Paris, so would it work if I buy a ticket from Frankfurt to Forbach then stay on the ICE and just use my Interrail-Ticket from Forbach to Paris on the same train?

Yes, but you would also need a reservation to go along with the Interrail Forbach - Paris. This may be €10 or €20.

Check passholder reservations for French TGV trips here https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

To book you need a pass cover number, printed on paper pass cover or for mobile pass get number here

The usual caveat applies to also check if a cheaper trip is available by just buying a Frankfurt - Paris fare from DB

Actually, only to Forbach(fr), which is the border tariff point here. But since DB's saver type tickets (Sparpreis etc.) are only available to real stations, it may be cheaper to buy a ticket to Forbach instead.

Thanks heaps for all your efforts. I think I am well prepared now to start my trip.