Forget sleeping in the car during Oktoberfest in Munich. Germans are pretty law abiding and will report you sleeping in their neighbourhood. There are temporary camping grounds set up in the fairgrounds, but you have to pay an entrance fee. Another option with the train is to take the night trains, that way you get sleep and travel at the same time. If you buy a Eurail pass, you just pay 20€/Person for a bed on the train.

In the bavarian countryside people do speak English if there is no other options. I have guests visiting who do just fine and friends who live in Munich since years without speaking the language.