As jspira indicated, I also drive so that I'm not commited to someone's schedule.

You should consider the following.

Berlin and Munich are very far. Have you consider doing an open jaw and flying into one city and out the other city?

Are both you and your GF drivers?

Gas is relatively expensive. Diesel to cover 900km cost me $150USD on my trip last week but I was flying down the Autobahn at 180 to 250 km/h in a Jag.

If you want to stay in a hotel look into B&B (www.hotelbb.de) or Motel One.

As for language, download an offline translator onto a smartphone.