Yes you can, Sixt has plenty of mostly German plated cars scattered all over Europe. I rented from Nice several times and got German and Dutch registered cars that I took across the border into Italy and back to France.

Though you need to ensure the vehicle class you're renting is good to be taken across borders because as you head deeper into Eastern Europe Sixt has restrictions on vehicle classes you're allowed to cross borders with (theft issues). Insurance in some places (Italy notably) is also something you need to think about in case you decline Sixt's coverage because most credit card affiliated insurance policies exclude a handful of countries.